Noonan Syndrome Presenting With Severe Transient Thrombocytopenia and Hemolytic Anemia in a Neonate With a Germline <i>PTPN11</i> Pathogenic Variant

ABSTRACT Noonan syndrome (NS) is a RASopathy that can have hematological presentations, most commonly NS‐related myeloproliferative disorder (NS/MPD) and juvenile myelomonocytic leukemia. Other neonatal hematologic presentations remain poorly defined. We describe a neonate presenting with profound thrombocytopenia and hemolytic anemia, extensive evaluation excluding immune, infectious, and thrombotic microangiopathic etiologies. Subsequent trio whole‐genome sequencing identified a pathogenic heterozygous germline PTPN11 variant, confirming NS. The variant, NM_002834.5 c.794G&gt;A (p.Arg265Gln), was inherited from an apparently asymptomatic parent. The patient had complete hematologic recovery over the first year of life. This case broadens the hematologic phenotype of NS and underscores the importance of considering RASopathies in neonatal cytopenias. Early molecular diagnosis informs management and surveillance in early childhood.