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. The gene discussed is RAP1B; the disease is Thrombocytopenia.