About 15 percent of SCID cases are caused by deficiency of adenosine deaminase (ADA), an enzyme required for the salvage of nucleotides within lymphoid cells. The lack of ADA causes the accumulation of toxic metabolites of adenosine (deoxy-adenosine and deoxy-ATP) within lymphoid cells, resulting in their demise. ADA deficiency results in profound lymphopenia affecting T cells, B cells, and NK cells. Rarely, mutations of ADA causing milder forms of enzyme deficiency lead to a milder form of combined immunodeficiency presenting at a later stage in life. Purine nucleoside phosphorylase (PNP) is an enzyme required for purine salvage within lymphocytes, and PNP deficiency causes a milder phenotype of SCID than seen in ADA deficiency. SCID due to PNP deficiency not treated with HSCT is fatal in childhood.