Fig. 1. MMC-induced chromatid interchanges. Examples of MMC-induced chromatid interchanges as observed in human lymphocytes using FISH combining whole chromosome and band-specific probes. (A) Metaphase spread containing a quadriradial chromatid exchange between chromosome 9 (red) and an unpainted chromosome (blue) with the exchange breakpoint at the paracentromeric heterochromatic band of chromosome 9 (green). Two unaffected chromosomes 8 (green, with red paracentromeric euchromatic band) are also present. (B) The most commonly observed quadriradial between the homologues of chromosome 9 (red) with both exchange breakpoints within the heterochromatic bands (green). (C) An exchange involving the heterochromatic band (green) of chromosome 1 (red) and an unpainted chromosome. (D) The heterochromatic bands of chromosomes 1 (green) and 9 (red) are involved in a chromatid exchange. The black and white images of the DAPI counterstained chromosomes were included to visualize the exchanges at higher contrast.