Somatic mosaicism is the accumulation of mutations in DNA sequence or copy number in cellular genomes after fertilization (typically in the form of copy number variations, single nucleotide mutations, new retrotransposon insertions). Somatic mosaicism has been detected in both normal cells and in various diseases. Somatic variations have been suggested to play a major role in driving neuronal diversity and genome evolution. We have demonstrated and quantified widespread somatic mosaicism in normal human fibroblasts (Abyzov et al, Nature, 2012; Abyzov et al, Genome Research, 2017) and in the fetal in human brain (Bae et al, Science, 2018). We are part of the Brain Somatic Mosaicism network, sponsored by the National Institute of Mental Health (NIMH) and encompassing laboratories from several major centers in the US.