Productive and nonproductive V O J O gene rearrangements from individual peripheral blood B cells were analyzed for the pattern and distribution of mutations. The eight RGYW motifs and their inverse repeats, WRCY, were present in germ-line V O genes significantly more often than anticipated by random chance (1.6-fold and 1.4-fold, respectively) and were also mutated in nonproductive rearrangements significantly more often than expected, with a frequency 1.96 fold greater than that of non-RGYW/WRCY motifs. As a result, 50 % of all mutations in nonproductive V O J O rearrangements occurred in RGYW/WRCY motifs. Each RGYW tetramer and its corresponding WRCY contained mutations at comparable frequencies. Furthermore, mutations of G and C were significantly more frequent in RGYW/WRCY but not in other tetranucleotides. Finally, mutations in codons contained within RGYW/ WRCY were significantly more frequent in complementarity-determining regions but not framework regions of productive compared to nonproductive rearrangements and were increased by a factor that was significantly greater than for mutations in other motifs. These results indicate that the mutational machinery targets the overrepresented RGYW motifs in V O genes on both DNA strands and that the resulting replacement mutations are preferentially selected into the productive repertoire.