Here we presented a model of fixation of a CSF3R truncation mutant in the transition from an inherited neutropenia to sMDS: from the expansion phase in the prenatal hematopoietic tissues, to initiation of the G-CSF treatment, to expansion of the mutant, and to replacement of the normal bone marrow by the pre-leukemic mutants. This evidence concerns the gene CSF3 and spondylometaphyseal dysplasia, Sedaghatian type.