The defect is a type of purine nucleoside phosphorylase deficiency with inactive phosphorylase. This results in an accumulation of deoxy-GTP which inhibits ribonucleotide reductase. Ribonucleotide reductase catalyzes the formation of deoxyribonucleotides from ribonucleotides. Thus, DNA replication is inhibited and cells cannot replicate.
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^Nezelof C, Jammet ML, Lortholary P, Labrune B, Lamy M (Oct 1964). "Hereditary Thymic Hypoplasia: Its Place And Responsibility In A Case Of Lymphocytic, Normoplasmocytic And Normoglobulinemic Aplasia In An Infant". Archives francaises de pediatrie21: 897–920. ISSN0003-9764. PMID14195287.