Under the category Science, select the subcategory Biology and then the sub-subcategory Molecular Biology. This will lead you to an alphabetical list of Web sites. This site lets you search eight Web databases simultaneously for Web pages containing the term leptin and to view the results as a single collated list. You choose which ones to search for leptin. Many of the search engines' robot programs will automatically find your site and include it in the database.
Subject directories are more selective and oft en apply certain criteria for inclusion.
You can also submit your site for incorporation into the catalogs and databases. Each site has a form to submit your URL for inclusion.