Two distinct Id2 cDNA clones, Id2A and Id2B (inhibitor of differentiat
ion), have been isolated from a human heart cDNA library and sequenced
. The deduced amino acid sequences show nearly complete identity, exce
pt for the nonsense mutation at codon 37 in Id2B. Characterization of
the genomic clones revealed that Id2B is encoded by an intronless gene
which probably arose after the evolutionary divergence of rats and hu
mans.