TdT/occult microhomology model for genesis of NPM1 mutations. (A) Type A mutation, single base of occult microhomology. (B) Type D mutation, 2 nucleotides added, the first visible as an N-nucleotide, the second used for occult microhomology. (C) Type B mutation, 3 nucleotides added, 2 visible as N-nucleotides. (D) c.863_864insCCGG mutation, 5 nucleotides added, 4 visible as N-nucleotides.