PSMB5 and neoplasm: Second, the constitutive β5 subunit is structurally altered in all 3 bortezomib-resistant tumor cell lines due to mutations in the PSMB5 gene introducing single amino acid substitutions (e.g. Ala49Thr) in the bortezomib-binding pocket leading to diminished bortezomib binding efficiency [8,9].