ABCB1 and malignant colon neoplasm: When implanted subcutaneously into SCID mice, undifferentiated signet ring carcinomas are produced by HT29 MDR1-negative cells, while tumors with tubular structures, but without signet ring cells, are formed by HT29 MDR1-positive cells (MDR1 overexpression in HT29 colon cancer cells grown in SCID mice) (19).