Dihydrofolic acid (conjugate base dihydrofolate) (DHF) is a folic acid (vitamin B<sub>9</sub>) derivative which is converted to tetrahydrofolic acid by dihydrofolate reductase. Since tetrahydrofolate is needed to make both purines and pyrimidines, which are building blocks of DNA and RNA, dihydrofolate reductase is targeted by various drugs to prevent nucleic acid synthesis.