We observe that faster mCA clonal expansion rate—higher PACER score—was associated with increased measured erythrocyte counts among individuals with mCAs or somatic single nucleotide variants affecting JAK2. As the presence of an elevated erythrocyte count in combination with mutations in JAK2 suggest a diagnosis of polycythemia vera, our observation suggests that mCA expansion rate in clones with lymphoid driver mutations may have utility in prognosticating risk of progression to hematologic malignancy31. This evidence concerns the gene JAK2 and acquired polycythemia vera.