Entrez allows you to retrieve molecular biology data and bibliographic citations from the NCBI's integrated databases. These include:
DNA sequences from GenBank, EMBL, and DDBJ
Protein sequences from SwissProt, PIR, PRF, PDB, and translated protein sequences from the DNA sequence databases
Genome and chromosome mapping data
Three-dimensional protein structures derived from PDB and incorporated into NCBI's Molecular Modelling Database (MMDB)
PubMed bibliographic database containing citations for nearly 17 million biomedical articles from the National Library of Medicine's MEDLINE and pre-MEDLINE databases