The mMCB studies were performed on a series of bone marrow aspirates from AML patients who were karyotypically normal on routine cytogenetics. In one of the studied cases with an AML-M6 (case 4 of Table 1), four out of 18 metaphase plates showed that for one chro­mosome 1 the chromatids of the p- and the q-arm were located side-by-side rather than aligned as expected (see Fig. 1). This anomaly was called a ‘torsion’ and was ex­plained by a rotation of 180? within the centromere. A more detailed evaluation of mMCB in 50 metaphase plates of case 4 showed torsions in all meta- and submeta­centric chromosomes but not in acrocentric ones.

To study this anomaly in more detail, two-color FISH studies using arm-specific probes for chromosomes 1, 9 and 16 were chosen. A torsion of chromosome 1 was de­tected in 18 out of 124 cells (14.5%) in patient 4, and in 2.4 to 6% in the other seven AML-M6-patients. Torsions of chromosomes 9 and 16 were detected in 2 to 10% and 4 to 10%, respectively, in patients 1 through 8. In patient 10 (AML-M2), patient 14 (AML out of MDS) and 13 (AML-M5), torsions of chromosomes 1, 9 and 16 were detected in comparable rates. Torsions were absent in bone marrow samples derived from patient 15 (leuko­penia) and patient 18 (thrombocytemia), and were com­pletely undetectable in peripheral blood lymphocytes of patients 19 through 24, fibroblasts of patients 25 through 27, amniocytes of patients 28 through 30, and the CV sample of patient 31. No torsions were detected in patients 9 (AML-M1) and 12 (AML-M4), however, here only 20 or 30 metaphase plates were available. To determine whether torsions are associated with AML, marrow aspi­rates derived from seven patients with rheumatoid arthritis (patients 32 through 38) were also analyzed. In all these, as well as in patient 17 (neutropenia from toxic bone mar­row impairment) and patient 16 (autoimmune disease), torsions were present in 0.5 to 3.5%.