Multitude multicolor banding (mMCB) studies on bone marrow cells of a case with acute myelogeneous leukemia (AML-M6) revealed an unusual condition of one of the homologous chromosomes 1 in four out of 18 metaphase plates: the chromatids of the p- and the q-arm were located side-by-side rather than in the usual tandem alignment. This anomaly was called a “torsion”, and was explained by a rotation of 180? within the centromere. More detailed studies by two-color fluorescence in situ hybridization (FISH) on 42 specimens showed that eight cases of AML-M6, two cases of AML-M2 and one case each with AML out of MDS (myelodysplastic syndrome) and AML-M5, torsion of chromosomes 1, 9 and 16 was present in 2-14% of the cells. However, torsions were absent in bone marrow derived from one patient who had leukopenia, one patient with autoimmune thrombocytemia and one with AML-M4, as well as in samples of periph­eral blood lymphocytes (six samples), fibroblasts (three samples), amniocytes (three samples) and one chorionic tissue. As torsions were also present in 1.0-3.5% of meta­phase plates of bone marrow samples derived from seven different patients with rheumatoid arthritis, one may sug­gest that they are bone-marrow specific artifacts induced during preparation of the cells. Torsions could only be observed in 24-hour/colcemid cultures of cells. The signif­icance of the torsion-anomaly here described requires further studies.

Key words: Chromosome torsions; Multicolor band­ing (MCB); Fluorescence in situ hybridization (FISH)