We have recently proposed in this journal that post-zygotic somatic mutations in SNCA, occurring in early embryogenesis, could contribute to PD, although our pilot study was negative.4 Somatic mutations can lead to the presence of more than one genetically distinct cell in a single organism (mosaicism)5 and could be absent in peripheral lymphocyte DNA, which is usually analyzed. The gene discussed is SNCA; the disease is Parkinson disease.