Therapeutic proteasome inhibitors bortezomib, carfilzomib and ixazomib also increase A3A cytidine deaminase activity, transcripts and protein abundance in both breast cancer and multiple myeloma (the cancer type most commonly treated with clinical proteasome inhibitors) cell lines, indicating that clinical use of these drugs may unintendingly increase A3A-mediated DNA damage and mutation. This evidence concerns the gene APOBEC3A and cancer.