In human and mouse, the germline contains a tandem array of highly homologous variable (V) gene elements which encode part of the antigen-binding region of the antibody protein. During evolution this array apparently arose by gene duplication followed by diversification of duplicated genes via point mutation and recombination. Analysis of germline V gene sequences using a novel algorithm shows that major recombination sites coincide with the borders of the leader intron and the cap site, consistent with the hypothesis that over evolutionary time cDNA derived by reverse transcription of pre-mRNA in B lymphocytes has recombined with germline DNA.