The human genome contains two related but non-identical orthologues of the yeast NER gene RAD23. The proteins encoded by the human genes are called RAD23A and RAD23B (human homologue of RAD23).

When purified from human cells, XPC is tightly complexed with HHRAD23B. In vitro, NER can proceed in the absence of HHRAD23B, but the efficiency of the in vitro reaction is considerably improved by its presence.

Deletion of both Hhrad23 genes in mice (but not either alone) results in embryonic lethality, indicating that in addition to their roles in NER, the mammalian HHRAD23A and HHRAD23B proteins have unknown redundant functions that are indispensable for cellular viability.

A third protein called centrin2/caltractin1, present in the centrosome of several organisms, has recently been identified in the XPC/HHRAD23B complex and shown to aid the stabilization of XPC protein by RAD23. This association raises interesting possibilities for regulatory relationships between NER and cell division.

See also

 NER system (nucleotide excision repair)