Somatic mosaicism is the increasingly recognised phenomenon where there are two populations of cells that differ genetically in the same individual. When one considers that there are approximately 1014 cells in the adult human body, about 30 000 genes and approximately 10⁻⁶–10⁻⁷ mutations per gene per cell division, it follows that all of us are mosaic at some level, for some genes. Recently, two reports, by Hes et al1 in this issue (see page 71) and by Aretz et al,2 demonstrated that somatic mosaicism can be found in about 20% of sporadic cases of familial adenomatous polyposis coli (FAP) in which mutation of APC can be found. Genetic testing of the affected individuals revealed mutation, but at a level that accounted for less than all copies of the implicated allele, suggesting that some cells and/or tissues had the mutation while others did not.

Mosaicism of NF1 and NF2 has been reported several times.3 Mutations are only found in pathologically affected tissues, and the clinical features and course of disease are often milder than usual.4 5 This is presumably because not all the susceptible tissues have inherited the causative mutation. Mosaicism has also be reported in Marfan syndrome, tuberous sclerosis, achrondroplasia, Duchenne muscular dystrophy and retinoblastoma among many other conditions.6 7

The point in embryological development at which a mutation arises has profound consequences for the individual and the family. If it arises in an essential domain of the APC gene in a single colonic epithelial stem cell, the only consequence will probably be adenomatous polyps in the segment of the colon that eventually becomes populated with the descendants of that single stem cell (eg, as hypothesised in the discussion of patient 7 in the series presented by Hes et al1 …