A complete lack of PNP activity results in severe immunodeficiency syndrome (SIDS) [64] but it is important to note that heterozygotic parents of PNP‐deficient patients with only 20%–30% of normal PNP activity are healthy [65]; and, as shown previously [66], only 8%–11% PNP activity is required for normal neurological and immune function. Here, PNP is linked to sudden infant death syndrome.