In 130 IBD patients treated with AZA/6-MP, the white blood cell count was significantly lower in the patients with the MRP4 variant alone (n = 26) than in patients with a wild allelotype (n = 74) (P = .014) or in the patients with the ITPase variant alone (n = 22) (P = .0095). Here, ABCC4 is linked to inflammatory bowel disease.