Examples of replication showing (A) normal DNA replication, (B) microhomology resulting in mispriming and formation of an internal tandem duplication (ITD), (C) nontemplated addition of nucleotides by TdT and the use of the terminal nucleotide to create an occult single base mispriming event to create an ITD with filler sequence, and (D) a similar occult mispriming event leading to an NPM1 four base pair insertion. (From Vassiliou GS. The curious incident of TdT-mediated mutations in AML. From Vassiliou GS. The curious incident of TdT-mediated mutations in AML. Blood 2019;134:2229-2231.