Uracil and its 5-methyl derivative; thymine, are biologically öf great importance in fundamental metabolism. They derive their importance because certain of their derivatives are building blocks of RNA and DNA., 5-Fluorouıacil is particularly effective in the treatment of advan- ced carcinoma especially of the breast and the gastrointestinal tract; thiouracil, used in treating hyperthyroidism and angina pectoris.