24-colour chromosome painting was performed by the 'spectral karyotyping' (SKY) technique, essentially as described (Schrock et al., 1996; Roberts et al., 1999).
Briefly, metaphases were hybridised with chromosome paints for all the chromosomes simultaneously, each chromosome being labelled directly or indirectly with a distinct combination of up to five fluorochromes: fluorescein, Spectrum Orange, Texas Red, Cy5 and Cy5.5.
Metaphases were imaged on a fluorescence microscope to show all five fluorochromes simultaneously, and the fluorescence analysed using a spectrometer and CCD camera (Spectracube, Applied Spectral Imaging). DNA was counterstained and imaged separately with DAPI (4,6-diamino-2-phenylindole).
The supplied software analyses the spectrum of each pixel of the image, determines which dyes are present and hence which chromosome. Each pixel of the image is then presented false-coloured in a 'classification colour' to show which chromosome spectrum best matches the spectrum recorded.
Representative karyotypes were assembled as follows: all rearranged chromosomes present in at least two metaphases were listed, and the number of copies of each chromosome was the mode.
FISH - Fluorescence in situ hybridisation
Chromosome copy numbers and translocated fragments were verified in a number of additional metaphases analysed by conventional 2- or 3-colour chromosome painting using separate fluorescent dyes for each chromosome paint.
Individual chromosome paints and centromeric repeat probes (prelabelled, from Oncor), were labelled and hybridised as described (Courtay-Cahen et al 2000). For 3-colour analysis FITC, Spectrum Orange (or Cy3) and Cy5 were used.