Figure 4: Megakaryocytes with hyperlobulated nuclei may be present in patients with a JAK2 exon 12 mutation, yet not be positive for the mutant allele. (A) Clusters of two or three megakaryocytes, many of which had atypical nuclear morphology, were identified in the bone marrow sample of a patient with a JAK2N542-E543del mutation. (B) Individual megakaryocytes were purified by laser capture microdissection, and genotyped. Only a proportion of the cells within a single cluster were positive for the JAK2 exon 12 mutation. The location of the six deleted bases (marked by diamonds) is provided to highlight the relevant mutation within JAK2 exon 12.