In an unusual case of myeloproliferative disease, the Ph1 chromosome was found in association with persistently elevated levels of LAP activity. Clinical findings in this case included marked thrombocytosis, basophilocytosis, absence of splenomegaly and a preceding history of untreated ankylosing spondylitis. Cytogenetic findings were compatible with the existence of the Ph1 chromosome in erythroid and megakaryocytic as well as granulocytic marrow precursors.

This case illustrates the difficulties currently encountered in the clinical differentiation of myeloproliferative disorders and in interpreting the diagnostic significance of the Ph1 chromosome. The co-existence in this case of the Ph1 chromosome and elevated LAP does not support the concept of a direct relationship between Group G chromosomes and LAP activity.

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