Question: In humans, several different genetic variants of the gene encoding terminal deoxynucleotidyl transferase (TdT) have been identified. A set of six variants, each with a single amino acid substitution in the protein, were studied, and were found to vary in their enzyme activity measured in vitro, in some cases by more than twofold. When nucleotide sequences of rearranged immunoglobulin genes from cells expressing different TdT variants were analyzed, the data showed that cells with