PSMB4 and cancer: Furthermore, co-amplification of PSM genes located in close proximity to one another (e.g. PSMB5 and PSMB11, 14q11.2; PSMB4 and PSMD4, 1q21.3; PSMB8 and PSMB9, 6p21.32) or known cancer drivers (e.g. co-amplification of ERBB2 and PSMB3) were also frequently amplified together.