A clonal analysis of hematopoiesis in a hypoplastic MDS patient was performed. Monoclonal utilization of PGK gene on her two X chromosomes was detected by Southern blot analysis of the bone marrow cells using a PGK probe after methylation-sensitive restriction enzyme treatment. This is compatible with monoclonal construction of her hematopoiesis, confirming MDS nature with a progression of her morphological dysplasia in bone marrow cells. RFLP of PGK gene on X chromosome is thus useful to detect a monoclonality in hematopoietic diseases.