In response to a stalled replication fork, SHPRH polyubiquitinates lysine-164 of PCNA that has already been monoubiquitinated on lysine-164 by RAD18:UBE2B (RAD18:RAD6) (Unk et al. 2006, Motegi et al. 2006, Motegi et al. 2008). The ubiquitin donor is the E2 complex UBE2N:UBE2V2 (UBC13:MMS2) containing ubiquitin conjugated to UBE2N. The resulting polyubiquitin chain contains lysine-63 (K63) linkages and appears to change the repair process from translesion synthesis (TLS) to template switching (TS). SHPRH interacts directly with PCNA, RAD18:UBE2B, and UBE2N:UBE2V2.