A small acrocentric chromosome, identical in appearance to the Philadelphia chromosome of chronic myelocytic leukemia, was identified in all karyotyped marrow cell metaphases prepared from a 74-yr-old male with refractory dysplastic anemia. The abnormal chromosome was shown by quinacrine fluorescent staining to be derived from a Y chromosome which has lost the characteristic brilliantly fluorescent material of its long arms. A normal Y chromosome was consistently observed in karyotypes of cultured peripheral blood lymphocytes from this patient.

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