Immunoglobulin V domains can be divided into eight unique segments, each of which plays a separate structural role in the creation of an antigen binding site. Three of these segments encode the VH/VL core and are preserved in all V domains. V family identity depends on sequence similarity in two segments which provide support for the antigen binding site. Within a family, gene segments pnmarily diverge in CDR1 and CDR2. H chain CDR3, flanked by H chain CDR1 and L chain CDRs 2 and 3, builds specificity at the center of the antigen binding site. These findings provide a structural context for interpreting the preferential use of V gene segments in specific immune responses. © 1994 Academic Press. All rights reserved.