Purine nucleoside phosphorylase (PNP) cleaves nucleosides by phosphorylating ribose to produce ribose-1-phosphate. PNP metabolizes adenosine, inosine and guanosine. PNP metabolizes the waste molecules deoxyinosine and deoxyguanosine which are formed when DNA breaks down. PNP deficiency results in severe combined immunodeficiency.