Nextclade Web

Nextclade Web is available online at This is the easiest way of using Nextclade and is the recommended way to get started.

The application accepts sequence data in FASTA format, performs alignment, mutation calling, clade assignment, phylogenetic placement and quality control checks and displays the results in tabular form as well as in the form of the phylogenetic tree. The results can also be downloaded as files, for further review and analysis.

Nextclade is built for quick feedback. The entire analysis, depending on the number of sequences to be processed, takes from a few seconds to a few minutes.

Despite being made in the form of a website, Nextclade runs its processing entirely offline. The algorithms are executed within your browser and the data never leaves your computer (i.e. no data upload is happening). Nextclade however still needs internet access to download its own modules and dataset files.

Getting started

Open in your browser.

⚠️ For the best experience we recommend using latest versions of Firefox or Chrome web browsers on a desktop computer or a laptop with a 1080p display or above.

Using Safari browser is not recommended due to its poor support of required web technologies.

For a demonstration of capabilities of Nextclade click on “Show me an example” link.

Show me an example

In order to run the analysis on your own data, prepare a FASTA file with your sequences and drag & drop it onto the “upload” area marked “Sequences”. The analysis starts immediately and you will be redirected to the results page.

Power users might want to switch to Advanced mode in order to get access to more configuration. This mode is equivalent to using the Nextclade CLI, and accepts the same input files and the same output files.

💡 There are other options of providing input data to Nextclade, including:

  • Dragging a file onto the “upload” area

  • Picking a file from computer storage: click “Select a file”

  • Providing a URL (link) to a file publicly available on the internet: click “From URL” tab

  • Pasting sequence data from clipboard: click “Paste” tab

  • Providing a URL using URL parameters


Nextclade analyzes your sequences locally in your browser. That means, sequences never leave your computer, ensuring full privacy by design.

⚠️ Since your computer is doing all the computational work (rather than a remote server), it is advisable to analyze at most a few hundred of sequences at a time, depending on your computer hardware. Nextclade leverages all processor cores available on your computer and might require large amounts of system memory to operate. For large-scale analysis (thousands to millions of sequences) you might want to try Nextclade CLI instead.

The analysis pipeline comprises the following steps:

  1. Alignment: Sequences are aligned to the reference genome using its custom Nextalign alignment algorithm.

  2. Translation: Nucleotide sequences are translated into amino acid sequences.

  3. Mutation calling: Nucleotide and amino acid changes are identified

  4. PCR primer changes are computed

  5. Phylogenetic placement: Sequences are placed on a reference tree, clades assigned to nearest neighbour.

  6. Quality control: Quality control metrics are calculated

You can get a quick overview of the results screen in the screenshot below: Results overview

QC metrics

Nextclade implements a variety of quality control metrics to quickly spot problems in your sequencing/assembly pipeline. You can get a quick idea which of your sequences are having problems by sorting the results table from bad to good ( click on the upper arrow in the column QC). Bad sequences are colored red, mediocre ones yellow and good ones white. You can view detailed results of the QC metrics by hovering your mouse over a sequences QC entry:

QC hover

Every icon corresponds to a different metric.

At the moment, there are 6 different quality control metrics implemented:

  • Missing data: Number of Ns in the sequence. Up to 300 is not penalized, sequences with more than 3000 Ns are considered bad.

  • Mixed sites: Number of bases with ambiguous nucleotide characters. Since mixed sites are considered indicative of impurities, already 10 mixed sites give a bad score.

  • Private mutations: Mutations that are additional to the nearest neighbouring sequence in the reference tree. Up to 8 private mutations are not penalized. 24 private mutations are considered bad.

  • Mutation Clusters: A mutation cluster is defined as more than 6 private mutations occurring within a 100 nucleotide window. 2 mutation clusters are considered bad.

  • Frame shifts: Number of insertions or deletions that are not a multiple of 3. 1 frameshift is considered mediocre, 2 frameshifts are bad.

  • Stop codons: Number of stop codons that occur in unexpected places. 1 misplaced stop codon is considered mediocre, 2 stop codons are bad.

Table data

Nextclade automatically infers the (probable) clade a sequence belongs to and displays the result in the table. Clades are determined by identifying the clade of the nearest neighbour on a reference tree.

The result table further displays for each sequence:

  • “Mut.”: number of mutations with respect to the root of the reference tree

  • “non-ACGTN”: number of ambiguous nucleotides that are not N

  • “Ns”: number of missing nucleotides indicated by N

  • “Gaps”: number of nucleotides that are deleted with respect to the reference sequence

  • “Ins.”: number of nucleotides that are inserted with respect to the reference sequence

Hovering over table entries reveals more detailed information. For example, hovering over the number of mutations reveals which nucleotides and aminoacids have changed with respect to the reference. Changes in bases that are used by common primers are also displayed.

Mutations tooltip

Alignment viewer

To the right of the table you can see the alignment with mutations and regions with missing data highlighted in color. You can quickly check how segments of missing data are distributed on the genome - whether it’s a few big chunks clustering in one area or many small missing segments.

Alignment view

You can zoom into a gene by clicking on the respective gene at the bottom.

Select Gene

In sequence view, one can observe mutations in a particular gene. One of Nextclade’s strengths is that nucleotide and amino acid changes are visualised in the tooltip in a codon-aware way as you can see in the example below

Alignment tooltip


In order to assign clades to sequences, Nextclade places all new sequences on a a reference tree. You can view the resulting tree by clicking on the tree icon at the top right.

The tree is visualized by Nextstrain Auspice. By default, only your sequences are highlighted. One limitation to be aware of is that new sequences are place one by one on the reference tree. Thus, no common internal nodes of new sequences are placed on the tree. If you are interested in seeing ancestral relationships between your sequences, we recommend you use Usher.

Tree with new sequences

Download data

Once Nextclade has finished its analysis, you can download the results in a variety of formats:

  • Detailed results in either json, tsv or csv format, containing most results such as clades, mutations, QC metrics etc.

  • Phylogenetic reference tree including new sequences in auspice.json format for viewing with Nextstrain Auspice

  • A fasta file containing the aligned sequences of all analyzed sequences

  • A fasta file containing translated and aligned peptide sequences of all analyzed sequences

  • A csv file of all insertions that have been stripped from the aligned sequences

  • A nextclade.errors.csv file containing all errors and warnings that occurred during the analysis, like genes that failed to be translated

  • A file containing all files mentioned above

URL parameters

Input files can be specified in the URL parameters. The name of the parameters match the corresponding --input* flags of Nextclade CLI and flags of the dataset get command for datasets.

You can learn more about input files and datasets in sections: Input files, and Nextclade datasets.

If input-fasta URL parameter is provided, Nextclade Web automatically starts the analysis after all input and dataset files are downloaded.

All parameters are optional.

URL parameter Meaning
input-fasta URL to a fasta file containing query sequences. If provided, the analysis will start automatically.
input-root-seq URL to a fasta file containing reference (root) sequence.
input-tree URL to a Auspice JSON v2 file containing reference tree.
input-pcr-primers URL to a CSV file containing PCR primers.
input-qc-config URL to a JSON file containing QC onfiguration.
input-gene-map URL to a GFF3 file containing gene map.
dataset-name Safe name of the dataset to use. Examples: sars-cov-2, flu_h3n2_ha
dataset-reference Accession of the reference sequence of the dataset to use: Examples: MN908947, CY034116.
dataset-tag Version tag of the dataset to use.

For example, the file with input sequences hosted at can be specified with:

(the newlines and the indentation are added here for readability, they should not be present in the URL)

In this case, Nextclade will download the default dataset (currently: SARS-CoV2 based on MN908947 reference), thill download the privided file, will skip the home page and will automatically start the analysis.

Multiple files can be specified, for example the sequences and the reference tree:

Another dataset can be specified with dataset-name:

Another dataset based on a particular reference sequence can be specified with a combination of dataset-name and dataset-reference:

💡 Nextclade is a client-side-only, single-page web application, hosted on a static server. We do not set any usage limits for the analyses triggered. Note that all the computation will happen on the end-user machine.

⚠️The linked resources should be available for fetching by a web browser on the client machine. Make sure Cross-Origin Resource Sharing (CORS) is enabled on your file server as well as that all required authentication (if any) is included into the file URL itself.

⚠️The URLs might get quite complex, so don’t forget to encode the special characters, to keep the URLs valid.

What’s next?

Congratulations, You have learned how to use Nextclade Web!

Going further, you might want to learn about the science behind the Nextclade internals in the Algorithm section.

For more advanced use-cases, check out Nextclade CLI.

Nextclade is an open-source project. We welcome ideas and contributions. Head to our GitHub repository if you want to obtain source code and contribute to the project.