In contrast to its physiologically low expression in normal adult tissue, except in the testes where it is highly expressed, aberrant expression of KDM5B has been demonstrated in skin, lung, prostate, bladder and recently in breast cancer tissues and cell lines, while KDM5B gene silencing, similar to knockdown of KDM3A, was shown to cause a significant G1/S transitional lag in MCF-7 breast tumor cells, suggesting its proliferative and tumorigenic activity [35–39]. The gene discussed is KDM5B; the disease is breast cancer.