Entrez is a set of tightly
linked databases, including nucleic sequences, protein sequences and MEDLINE. It has a very nice
interface and is a very powerful and useful system. It has links to
itself! In other
words when you find an interesting nucleic sequence entry, you can
quickly find others like it, the corresponding protein entry and abstracts of papers describing it . http://www.ncbi.nlm.nih.gov/Entrez/