RCHY1 (Pirh2) acts as an E3 ubiquitin ligase to monoubiquitinate POLH (DNA polymerase eta) on lysine residues K682, K686, K694 and K709 located in the NLS (nuclear localization signal) of POLH (Jung et al. 2011). The NLS sequence of POLH is located between UBZ domain and PIP box, involved in POLH binding to monoubiquitinated PCNA (MonoUb:K164-PCNA). POLH monoubiquitination masks the PCNA-interaction region, thus disabling POLH binding to MonoUb:K164-PCNA and preventing POLH-mediated translesion DNA synthesis (TLS) (Bienko et al. 2010).