T and B cells exploit the mechanism of V(D)J recombination to make diverse or very restricted repertoires at varying times during ontogeny. Fetal repertoires are limited since there are no N nucleotides. Also, if short sequence homologies are present near the coding ends, junctions are preferentially made at that site. For gamma delta TCR, and to a lesser extent for Ig, this results in a very homogeneous population of junctions early in ontogeny. alpha beta TCR, however, have a paucity of homologous stretches, and maintain junctional diversity in the newborn. In both newborns and adults, some coding ends show very restricted nucleotide deletion, while others show heterogeneous and extensive deletion. It appears that the sequences of the coding ends have been selected through evolution as a mechanism to control repertoire formation.