Third reported patient with RAP1B-related syndromic thrombocytopenia and novel clinical findings.

RAP1B is a RAS-superfamily small GTP-binding protein involved in numerous cell processes. Pathogenic gain-of-function variants in this gene have been associated with RAP1B-related syndromic thrombocytopenia, an ultrarare disorder characterized by hematologic abnormalities, neurodevelopmental delays, growth delay, and congenital birth defects including cardiovascular, genitourinary, neurologic, and skeletal systems. We report a 23-year-old male with a novel, de novo RAP1B gain-of-function variant identified on genome sequencing. This is the third reported case which expands the molecular and phenotypic spectrum of RAP1B-related syndromic thrombocytopenia.