During newborn screening for SCID, P2 was diagnosed with reduced sjTRECs and marked CD4 + (0.15 × 109/L; normal reference: 1.60–4.00 × 109/L) and CD8 + (0.03 × 109/L; normal reference: 0.56–1.70 × 109/L) T cell lymphopenia, prompting whole genome sequencing of proband (P2), parents (mother (P1) and father) and maternal grandparents, supplemented by a panel of 365 PID genes. Here, CD4 is linked to lymphopenia.