In adult animals, template-independent (or N) nucleotides are frequently added during the rearrangement of variable (V), diversity (D), and joining (J) segments of lymphocyte receptor genes, greatly enhancing junctional diversity. Receptor genes from adult mice carrying a mutation in the terminal deoxynucleotidyl transferase (TdT) gene have few N nucleotides, providing proof that this enzyme is essential for creating diversity. Unlike those from normal adults, receptor genes from adult mutant mice show extensive evidence of homology-directed recombination, suggesting that TdT blocks this process. Thus, switch-on of the TdT gene during the first week after birth provokes an even greater expansion of lymphocyte receptor diversity than had previously been thought.