Abstract
The urines of two unrelated children with inherited deficiencies of purine nucleoside phosphorylase have been found to contain significant quantities of orotic acid in addition to the previously reported purine nucleosides. The data are consistent with some cell types of these immunodeficient patients being deplete of pyrophosphoribosylphosphate, a precursor of both purine, and pyrimidine nucleosides. It is suggested that the pyrophosphoribosyl-phosphate-depleted cells may be some component of the thymus-dependent immune system.