It is perhaps more surprising that the f-Hb profile in those who had a negativecolonoscopy in the first round shifts to lower f-Hb in the second round, butthis may be explained by the fact that an episode of bleeding from a benign,self-limiting condition, such as haemorrhoids or acute diverticulitis, isunlikely to occur again in the second round at the exact time of specimencollection. This evidence concerns the gene GSTM1 and hemorrhoid.