CYP1A1 and carcinoma of esophagus: However, Nimura et al (1997) studied 89 oesophageal carcinoma patients and 137 cancer-free control patients in an ethnically Chinese population and reported that heavy smokers with Val/Val genotypes of CYP1A1 exon 7 had a three-fold risk of developing oesophageal cancer as compared to those with Ile/Ile genotypes.