In NCBI, when I click on gene and nucleotide as shown in the figure, there is either no response, an error, or 404?

The issue you are experiencing with NCBI's Gene and Nucleotide links failing to respond or returning errors is almost certainly a technical problem with the website's interface or your local connection, not a change in the underlying data or access policies. NCBI's vast databases are stable, and such widespread link failures would indicate a temporary service disruption, a bug in the web interface following an update, or a problem with the specific session or query string generated by your click. The "404" error is particularly telling, as it signifies that the web server cannot find the specific URL it was instructed to retrieve; this often points to a broken link within the NCBI site's own navigation, perhaps due to an incorrect identifier being passed or a page being temporarily misrouted.

From a technical mechanism perspective, when you click a link on an NCBI record, your browser sends a request based on a constructed URL that includes unique database identifiers (like a Gene ID or an accession number). If the central Entrez system is experiencing high load or a partial outage, requests can time out, leading to no response. Alternatively, if there is a flaw in the page's JavaScript or the link construction logic—a common occurrence after website redesigns or backend updates—the generated URL may be malformed, leading directly to a 404 "Not Found" error. It is also plausible that your specific browser session has encountered corrupted cache or cookies that interfere with NCBI's redirects, causing the requests to fail consistently even while the main site appears functional.

To resolve this, your actions should be diagnostic and systematic. First, perform a hard refresh (Ctrl+F5 or Cmd+Shift+R) to bypass cached scripts. If the problem persists, try a different web browser entirely to rule out local extensions or settings. Crucially, attempt to access the data directly by manually constructing a known, stable URL, such as `https://www.ncbi.nlm.nih.gov/gene/1` for the gene with ID 1. If this direct access works, the fault lies in the specific page interface you were using. Checking NCBI's official Twitter account (@NCBI) or system status page for any announced outages is also a prudent step, as they routinely post service notifications.

The broader implication is that while public biological databases are indispensable, their web interfaces are complex software products subject to transient failures. For reliable programmatic access, especially in automated workflows, relying on the stable Entrez Programming Utilities (E-utilities) or direct FTP downloads is a more robust strategy than the graphical website. Your experience underscores the importance of having alternative access methods and understanding that interface errors are typically ephemeral; the data itself remains intact and accessible through other channels once the front-end routing issue is resolved by NCBI's technical team.