Pericentromeric polymorphisms of chromosome 9 include variations in the size of heterochromatin, pericentric inversions, and, more rarely, additional C-band-negative, G-band-positive material in either the proximal short arm or long arm or within the heterochromatin. It has been postulated that rearrangements involving the different classes of satellite DNA present in this relatively unstable region of the human genome constitute a mechanism for the origin of these variants. We report the identification, by molecular cytogenetic investigations, of homologous stretches of euchromatin shared by the proximal short and long arms of chromosome 9 that suggest that exchanges involving these regions may be an additional mechanism for the origin of chromosome 9 polymorphisms.