Heterozygous germ line CSF3R T618I mutation in a child with CNL. (A) Serial blood counts by year showing white blood count (WBC), hemoglobin (HGB), hematocrit (HCT), platelet count, absolute neutrophil count (ANC), percentage of neutrophils, and percentage of bands. (B) Mutational analysis of genes frequently implicated in myeloproliferative neoplasms (MPNs) using next-generation and/or Sanger sequencing (WT, wild type; neg, negative). ¹V617F and exon 12 mutations were not detected. A single nucleotide polymorphism (SNP) in exon 9 (L393V) was detected. ²Mutations in ASXL1 were not detected. M1249V, a known SNP, was identified. (C) Using next-generation sequencing (NGS), the entire CSF3R gene was sequenced from genomic DNA (gDNA) from whole blood from the patient, the unaffected parents, and the unaffected sibling, as well as from gDNA isolated from a buccal swab from the patient. A heterozygous T618I mutation was identified in the gDNA isolated from both blood and buccal cells from the patient. CSF3R W791* was detected in blood cells only from the patient at low frequency (16%). (D) Confirmation of the CSF3R T618I mutation in patient cells in panel C by Sanger sequencing. (E) Patient pedigree showing the CSF3R T618I mutation present in the patient but undetectable in either parent or the unaffected sibling. Serum G-CSF levels were below normal in the patient but within the normal range in the parents and unaffected sibling.