Summary.A family is described in which six members developed acute nonlymphocytic leukaemia (ANLL) over the past 1 5 years, all morphologically FAB M6. Five affected individuals (ages 54‐73) are members of a single sibship, the sixth (age 31) was the son of one of the affected sibs. The family lives in a rural area, and no environmental hazard has been identified. One of the six patients never resided in the same geographic area as the others. Three patients with leukaemia died without chemotherapy, two died shortly after the initiation of chemotherapy and the most recent patient never achieved remission, surviving 10 months. Bone marrow specimens from the two most recent patients were analysed for cytogenetic abnormalities using chromosome banding techniques. In addition, skin fibroblasts were examined in a single patient. The fibroblasts had a normal karyotype. Bone marrow cells from both patients had complex abnormalities. Some similarities were observed, including: (1) hypodiploidy; (2) loss of one chromosome 20; (3) the presence of multiple marker chromosomes. In each case one marker appeared to involve the‘missing’no. 20 with an alteration of the long arm—loss (20q —) in one patient and gain (20q +) in the other. Peripheral blood lymphocytes were grown in folate deficient medium, but no evidence for folate sensitive fragile sites was found. Marrow chromosomes from a third leukaemic sibling were examined without banding techniques in 1976 and the karyotype was 45.XY, — 18, — F(19 or 20), ‐I‐marker. No specific cytogenetic finding was common to the three patients studied except for the loss of an F group chromosome. This family is unique in the literature with six cases of familial leukaemia of the same morphologic subtype, and the cytogenetic findings suggest that loss or rearrangement of part of the long arm of a single chromosome 20 may have occurred in each of the three
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