GSTM1 and colorectal carcinoma: However, although in this group as a whole there was no clear trend in f-Hb with age, there was a greater tendency for older subjects with no CRC or adenoma to have f-Hb of 20 μg/g or more: the proportions were 2.8%, 3.1% and 3.3% for age groups 59–64, 65–69 and 70–75 years, respectively (p = 0.045).