Template-independent DNA polymerase which catalyzes the random addition of deoxynucleoside 5'-triphosphate to the 3'-end of a DNA initiator. One of the in vivo functions of this enzyme is the addition of nucleotides at the junction (N region) of rearranged Ig heavy chain and T-cell receptor gene segments during the maturation of B- and T-cells. a 2'-deoxyribonucleoside 5'-triphosphate + DNA(n) = diphosphate + DNA(n+1) Can also utilize other divalent cations, such as Mn(2+) and Co(2+) (in vitro). Interacts with PRP19 and DNTTIP1. Forms a ternary complex with DNTTIP2 and core histone. Released from this complex by PCNA. Interacts with TRERF1 (PubMed:16371131). Belongs to the DNA polymerase type-X family.