The average value for Ki67 expression in the population of untreated SK-BR-3 cells was 61.68%, whereas in the ouabain-treated cells, Ki67 was expressed only in 19.31%, thus suggesting that a concentration of 10−6 M of the Na+/K+ pump blocker has an antiproliferative effect on SK-BR-3 tumor cells. The gene discussed is MKI67; the disease is neoplasm.