ABCG2 and neoplasm: Furthermore, the ABCG2 profile was significantly associated with progression to the higher grade of the tumor (R2 = 0.05, p = 0.007, r = 0.22), tumor grade (R2 = 0.07, p = 0.0008, r = 0.28), as well as Ki67 expression (R2 = 0.03, p = 0.04, r = 0.18).