The defect is an inactive purine nucleoside 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.
^Knutsen AP, Wall D, Mueller KR, Bouhasin JD (May 1996). "Abnormal in vitro thymocyte differentiation in a patient with severe combined immunodeficiency-Nezelof's syndrome". J. Clin. Immunol.16 (3): 151–8. doi:10.1007/BF01540913. PMID8734358.
^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.