Typical SCID is defined as a patient with: (a) mutation(s) in a gene associated with a typical SCID phenotype; or, (b) presentation with severe or opportunistic infections, persistent diarrhea and failure to thrive, in the presence of low (300/μL) or absent CD3+ or CD4+ or CD8+ T cells, reduced naive CD4+ (CD3+CD4+CD45RA+) and/or CD8+ (CD3+CD8+CD45RA+) T cells, elevated γδ T cells, absence of proliferative responses to mitogens, defined as proliferative response to phytohemagglutinin (PHA) lower than 10% of the control subject; or (c) T cells of maternal origin present. Here, CD8A is linked to Failure to thrive.