PDGFRB and Chronic Eosinophilic Leukemia, Not Otherwise Specified: Primary eosinophilic disorders are rare in the pediatric population and include chronic eosinophilic leukemia; myeloid/lymphoid neoplasms with rearrangements of PDGFRA, PDGFRB, FGFR1, or PCM1-JAK2; and idiopathic hypereosinophilic syndrome [2].