We identified a de novo heterozygous single nucleotide substitution c.35G>A in RAP1B, p.G12E, in a 2-year-old boy with an undescribed hematological disorder associated with CID, neutropenia, and monocytopenia as well as severe thrombocytopenia and platelet dysfunction. This evidence concerns the gene RAP1B and Decreased total monocyte count.