Additional small ring chromosome 1 is described with increasing rate of mosaicism in three generations. Ten years after the first examination, the mosaic rates in the patients were strikingly similar. An increase in the expression of phenotypic anomalies was also observed in the successive generations. FISH examinations following microdissection revealed signals which were positive for 1p13 and 1q21 indicating that the ring contained euchromatic segments on both ends. Additionally, array-CGH whole-genome analysis showed a single copy gain corresponding to band 1p12 to band 1q21-1 of chromosome 1 in the patients. The presence of euchromatic material from chromosome 1 in the ring suggests that the relationship between the cytogenetic findings and the clinical manifestation is likely causative. These unique observations might be explained by mitotic loss of the ring at early embryogenesis, and would indicate different mitotic vulnerability of certain chromosome abnormalities at early postzygotic stages versus later during development.