By contrast, treatment of breast cancer cells with rigosertib promoted the negative feedback phosphorylation of MEK1 at Thr286, Thr292, and Thr386 without affecting the positively regulated phosphorylation of MEK1 at Ser217/221 or ERK1/2 at Thr202/Tyr204. The gene discussed is MAP2K1; the disease is breast carcinoma.