USP5 and nasopharyngeal carcinoma: Moreover, immunofluorescent staining of γH2AX, a marker of DSBs, showed that USP5 knockdown alone did not result in γH2AX focus formation in the NPC cells (data not shown), whereas USP5 knockdown significantly increased irradiation-induced γH2AX focus number in the NPC cells, and EphA2 overexpression recovered irradiation-induced γH2AX focus number in the NPC cells with USP5 knockdown (Fig. 4C).