An integral part of the diagnosis of human leukemia is the cytogenetic analysis of bone marrow aspirate for specific acquired chromosomal abnormalities. Recurrent chromosomal aberrations result from events at the molecu­lar level and the affected chromosomal regions may con­tain genes that promote or cause the disease. Routine cyto­genetics has identified many important karyotypic changes that gave hints on the localization of oncogenes or tumor suppressor genes [1,2]. The introduction, in the last de­cade, of molecular cytogenetics has facilitated routine tumor cytogenetic analysis.

After standard chromosome banding (i.e., GTG-band­ing) in about 30 to 50% of patients with acute myelo­geneous leukemia (AML) no cytogenetic changes were detectable in the bone marrow cells [3]. As GTG- banding cannot detect all possible chromosomal changes [4,5], a search for additional cryptic chromosomal aberrations was carried out in a series of AML cases. These specimens, which were, according to GTG-banding results, cyto­genetically normal, were studied using the multitude multicolor banding (mMCB) technique [5]. During these studies, we came across an anomaly we called “chromo­some torsion”, i.e., the chromatids of the p- and the q-arm were located side-by-side rather than in the usual tandem alignment. Such torsions were previously seen during leu­kemia banding cytogenetic diagnostics (Dr. E. Gebhart, Erlangen, Germany, personal communication, 2004), how­ever, this has not thus far been studied in detail.