A 19bp deletion polymorphism of intron 1 of dihydrofolate reductase (DHFR 19bpdel) increases the risk for breast cancer, and retinoblastoma of the offspring, in folic acid supplement users. Folic acid is a synthetic form of folate present in fortified foods and supplements that must be converted to tetrahydrofolate by DHFR to enter the metabolism. Individuals homozygous for DHFR 19bpdel have higher prevalence of unmetabolized folic acid in plasma and lower incorporation of folic acid into tissues. How the DHFR19bpdel (17% homozygosity in US) affects DHFR activity and folate metabolism to increase cancer risk is not understood. Studies on this topic are urgent in the light of mandatory folic acid fortification in the US and other countries. The objective of this project is to characterize the effect of DHFR 19bpdel on DHFR activity and folate pathway reactions and to determine if the effect of DHFR 19bpdel can be alleviated with folinic acid, which is a folate source that need not be converted by DHFR. The specific aims of this project are to 1] Determine expression of DHFR mRNA and protein, splicing of intron 1 and enzyme activity in white blood cells from 3 DHFR genotypes. 2] Determine the effect of DHFR 19bpdel and folic acid or folinic acid concentration on cell growth, and folate pathway reactions in white blood cells in homozygotes for DHFR 19bpdel and those who lack the polymorphism. Results of this study will guide measures to reduce this modifiable cancer risk associated with DHFR 19bpdel.