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Human myosin light chain-2 (MYL2) is an important protein involved in the regulation of myosin ATPase activity in smooth muscle. In cardiac muscle, the precise role of MYL2 is not well understood; however, an increase in ventricular MYL2 is observed during myocardial hypertrophy in cardiac patients with valve stenosis. The chromosomal location of the gene coding for MYL2 was identified using a cloned cDNA for human MYL2. Southern blot analysis of DNA from a human/rodent somatic cell hybrid mapping panel showed that the BamHI fragment that hybridized with this cDNA probe was concordant with chromosome 12. The 768-bp cDNA was hybridized to human metaphase chromosomes. The results revealed a significant clustering of silver grains over chromosome 12 bands q23-q24.3, indicating that the gene coding for MYL2 is located in this region.

A patient with chronic myelogenous leukemia (CML) having the standard [t(9; 22), Ph] translocation is presented where the Philadelphia (Ph) chromosome disappeared following bone marrow transplantation (BMT). The Ph chromosome reappeared in host cells after one year of stable hematologic remission. Three additional cell lines, all possessing the Ph chromosome with other abnormalities were consistently present in her marrow cells. Two years after BMT, ninety percent of her dividing bone marrow cells had become leukemic. The patient's clinical status remains unchanged, despite complex cytogenetic findings. The high incidence of multiple aberrant leukemic clones present in this case remains intriguing. Possible mechanisms for this unique transformation after BMT are discussed.