COMT and breast cancer: However, a correlation between higher plasma cysteine levels and increased breast cancer risk was not observed in another prospective nested case–control study, wherein an inverse cysteine–cancer risk relationship was particularly evident in leaner women183 or related to the catechol-O-methyltransferase (COMT) genotype compatible with high enzymatic activity.185 Inconsistencies in the outcome of these studies might be attributed to the heterogeneity of the populations studied and/or different conditions of analysis and concentrations of plasma cysteine used.