More than 20 years have passed since terminal deoxynucleotidyl transferase (TdT) was first described and characterized as an anomaly among eucaryotic polymerizing enzymes. 1 The last several years have witnessed a resurgence of interest in the chemistry and biology of TdT as more diverse cellular sources were discovered. In the past, TdT has played an important role in physical and chemical studies of polydeoxynucleotides, 2 - 5 in studies of mechanisms of enzymatic DNA replication, 6 - 10 and in studies of UV damage in DNA. 11 - 13 Its current applications are as diverse as end-labeling of DNA molecules destined for recombination into plasmid vectors 14 - 16 and classification of acute leukemias as well as certain types of lymphoma. 17 - 21 TdT serves as a unique biological marker of primitive lymphoid cells and may contribute to their differentiation and even diversification. 22 - 27 Nevertheless, the precise biological function of this versatile protein remains an enigma.