The invention provides a method of increasing expression of p21.sup.WAF1/Cip 1 in cells to decrease proliferation of the cells, the method comprising decreasing levels of PP5 protein in the cells. The invention further provides a method of treating or preventing an abnormal condition resulting from a defect in a tumor suppressor gene in a subject that results in decreased induction of p21.sup.WAF1/Cip 1 in the cells of the subject, the method comprising administering to the subject an amount of a compound effective to decrease levels of PP5 protein in the cells of the subject.