XPC and lung carcinoma: Importantly, mice heterozygous in Xpc (Xpc+/-) exposed to the carcinogen, urethane, developed an intermediate number of lung tumors when to compared to urethane-treated Xpc deficient and proficient littermate mice, suggesting a gene-dose effect and further supporting a role for intermediate levels of XPC expression, either through polymorphisms or epigenetic regulation, in lung cancer development (42).