The pivaloyloxymethylester Pomstafib-2 (8) inhibits STAT5b phosphorylation in cultured human leukaemia cells with an IC50 of only 1.5 μM, without significantly affecting the phosphorylation of STAT5a, and increases the apoptotic rate of human leukaemia cells in a STAT5-dependent manner. The gene discussed is STAT5B; the disease is leukemia.