Cancer cells are more vulnerable to the effects of cardiac glycosides than normal cells because activation of the Src-epidermal growth factor receptor-mitogen-activated protein kinase pathway by cardiac glycosides in cancer cells, unlike in normal cells, results in growth arrest through an increased expression of the cyclin-dependent kinase inhibitor 1 A (p21CIP1)43. The gene discussed is CDKN1A; the disease is cancer.