PNP (Purine Nucleoside Phosphorylase) is a key enzyme involved in the metabolism of nucleotides, particularly in the recycling and degradation of purine and pyrimidine nucleosides. Expressed in a variety of cell types and tissues, PNP activity is crucial for maintaining the balance of nucleotides within cells. Abnormal PNP activity is associated with certain metabolic diseases and immune system dysfunctions. Specific expression and mutations of the PNP gene can affect its enzymatic activity, thereby impacting the metabolic state of cells and susceptibility to diseases.