Gene-disease-drug target associations.
Open Targets API is a free Health API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
Details from platform-docs.opentargets.org.

Unified API suite · now live
Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
Public FDA data about drugs, devices and foods
Medical platform which allows the development of applications for different healthcare scenarios
Coronavirus API with free COVID-19 live updates
The Open Targets API is provided by Open Targets Platform (platform-docs.opentargets.org).
Yes. The Open Targets API is listed in the Health 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 Open Targets API requires no authentication, so you can start making requests without signing up.
Yes. The Open Targets API supports CORS, so front-end JavaScript can call it directly.
Yes. The Open Targets API is served over HTTPS, so it's safe to use in production.