Many xenobiotics agents are metabolized by enzymes mechanisms through Phase I, activating substances procancerogene through oxidative reactions, and / or through mechanisms Phase II, acting on metabolic intermediate products of oxidative processes with conjugation reactions with endogenous mediators, in order to generate hydrophilic products that can be easily excreted by the body. Among the enzymes Phase II is a heterogeneous group represented by glutathione S-transferase. Genetic polymorphisms encoding for these enzymes (GSTs) are responsible phenotypic expression of enzymes specifically involved in the detoxification and elimination of different genotoxic agents (IPA, toluene, benzene). Accordingly, the authors have investigated a population of subjects professionally exposed to benzene (used in active refining and storage of crude oil) in order to assess the genetic profile in relation to possible null genotype (responsible for the failure phenotypic expression of protein) of polymorphism GSTT1 and GSTM1 and correlate the impact that the genotype effect of normal metabolic pathway t, t-muconico.