DNTT and type 1 diabetes mellitus: Alterations in the insertion patterns can result from mutations in exon 9 of the DNTT gene encoding the active site of the terminal deoxynucleotidyl transferase (TdT)54, although all T1D patients in the present study had the wild-type exon 9 sequence The balance of differentially spliced forms of TdT, which are known to differ in N-nucleotide addition and exonuclease activities55, could also impact upon insertion patterns.