banding analysis of the EPT1 cells (B) showed the same marker chromosome in all as found in the EPT156 cells (red arrow) but in addition a loss of the normal chromosome 20 (black arrow). Clonal evolution involving chromosome 13 (blue arrow) was also found. The composite karyotype can be described as: 46–47,XY,+13[3], +i(13)(q10)[2],der(20)[10],221[3][cp10]. In order to gain more information on the source of the marker chromosome FISH analysis (C) was performed. A probe against centromer 20 revealed that the marker chromosome was a chromosome 20 derivative (der(20), red arrow), whereas a probe against the subtelomeric region of 12q showed no involvement of that chromosome. Found at: doi:10.1371/journal.pone.0003368.s002 (2.76 MB TIF) Acknowledgments We are thankful to Beth Johannessen regarding cell culture work and RNA purification and to Hua My Hoang regarding RNA purification, nucleic acid labelling and microarray assays and to Kjetil Solland regarding References 1. 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