ABCB1 and neoplasm: When compared with the negative KB 3.1 and positive KB 8.5 control cell lines, 68 (42%) of tumours did not express the MDR1 gene, while 96 tumours (58%) expressed MDR1. The mean value of the MDR1/β2M ratio was 0.052±0.008 (range: 0–0.065), with a median of 0.02.