Purine nucleoside phosphorylase (PNP) deficiency leading to accumulation of lymphocytes in S-phase.

Freshly isolated mononuclear cells of a patient with purine nucleoside phosphorylase (PNP) deficiency contained 7-11% cells in S-phase. During treatment with deoxycytidine and tetrahydrouridine these cells disappeared from peripheral circulation, indicating that the in vivo accumulation of S-phase cells is caused by a shortage in deoxycytidine triphosphate. In vitro it was not possible to cause a blockade in S-phase by culturing normal or PNP-deficient lymphocytes in the presence of deoxyguanosine.