The NM_001754.5(RUNX1):c.166_196del (p.Leu56ProfsTer6) variant in RUNX1 is a frameshift variant predicted to cause a premature stop codon in biologically relevant exon 4/9 leading to nonsense mediated decay in a gene in which loss-of-function is an established disease mechanism (PVS1). This variant is absent from gnomAD v2, v3, and v4 with at least 20x coverage (PM2_Supporting). It has been reported in a proband with thrombocytopenia and MDS/MPN-U and her "affected" sister (PMID: 37216690), as well as in 2 additional probands meeting phenotypic criteria for RUNX1 (PMID: 37406166) (PS4_Moderate, PP1 is not met). It has also been reported in a patient with MDS, but variant origin is unclear (cBioPortal.org - Papaemmanuil Lab, 2022). Furthermore, other pathogenic/likely pathogenic frameshift alterations in exon 4 have been reported (PMID: 35764482) (PM5_Supporting). In summary, this variant meets the criteria to be classified as pathogenic for autosomal dominant hereditary thrombocytopenia and hematologic cancer predisposition syndrome based on the ACMG/AMP criteria applied, as specified by the ClinGen Myeloid Malignancy VCEP: PVS1, PS4_Moderate, PM2_Supporting, and PM5_Supporting. (Version 2; date of approval)