OMIM (Online Mendelian Inheritance in Man)
This database has the same content as OMIM (Online Medelian Inheritance in Man) (via NCBI). You do not need to search both places.
A comprehensive, authoritative, and timely compendium of human genes and genetic phenotypes. The full-text, referenced overviews in OMIM contain information on all known mendelian disorders and over 12,000 genes. OMIM focuses on the relationship between phenotype and genotype. It is updated daily, and the entries contain copious links to other genetics resources.