Nucleotide sequences, genomes and raw reads archive.
ENA API is a free Science & Math API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.

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Yes. The ENA API is listed in the Science & Math category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The ENA API requires no authentication, so you can start making requests without signing up.
Yes. The ENA API supports CORS, so front-end JavaScript can call it directly.
Yes. The ENA API is served over HTTPS, so it's safe to use in production.