Mutation was picked up following abnormal newborn screening for SCID in which TRECs were absent/very low. Initial immune investigations showed severe T-cell lymphopenia, particularly affecting CD4+ and CD8+ cells, leading to a diagnosis of T-B+NK+ SCID. The patient remained clinically well with watchful waiting and monitoring of immunological tests. Over the course of the first 18 months of life, CD4+ and CD8+ counts generally improved but remained low relative to normal ranges. The gene discussed is CD4; the disease is severe combined immunodeficiency.