Felgentreff et al. (5) recently reported immunological findings in two asymptomatic siblings who shared a LIG4 bi-allelic mutation (K449Q/R814X) with their sister (the proband), but not a history of infections; all three siblings with compound heterozygous mutations had: low mean corpuscular volume, B-cell lymphopenia, shortened telomeres, reduced naïve T-cells, and increased radiosensitivity, as well as some skewing of their lymphocytes' receptor repertoires. The gene discussed is LIG4; the disease is Decreased total B cell count.