Patient 1 JAK2 exon 13 insertion/deletion mutation and endogenous erythroid colony formation assay. (A) Structural layout of the JAK2 kinase from N terminus to C terminus. Critical domains are labeled in red. The amino acid sequences of the pseudokinase (JH2) domain of JAK2^WT, JAK2^V617F, and JAK2^ex13InDel are highlighted. Note the deletion of residues 583 to 586 in JAK2^ex13InDel and insertion of an in-frame serine residue. Tyrosine 114 in the FERM domain is critical for interactions with cytokine receptors. (B) Trends in the patient’s blood counts in response to ruxolitinib and 5-azacitidine treatment. BID, twice daily; Eos, eosinophil; Hgb, hemoglobin; Plt, platelet; WBC, white blood cell. (C) Peripheral blood mononuclear cells from the patient and a healthy control were cultured in the absence or presence of graded concentrations of EPO. In the absence of EPO, 10 EECs were observed to grow out from patient-derived mononuclear cells. Numbers above bars represent number of colonies. (D) Patient-derived EECs were plucked and genotyped using JAK2 allele-specific polymerase chain reaction followed by fragment length analysis. Seven of 8 patient-derived EECs that were genotyped exhibited heterozygosity for JAK2^ex13InDel. BFU-E, erythroid burst-forming units; WT, wild-type.