The NM_177438.2:c.5125G>A variant in DICER1 is a missense variant predicted to cause substitution of aspartic acid by asparagine at amino acid 1709 (p.Asp1709Asn). This variant received a total of 2 phenotype points across 2 unrelated probands meeting DICER1 VCEP phenotype specificity scoring criteria of 2-3.5 points (PS4_Moderate, PMIDs: 26925222, 26475046). In both probands, this variant was identified as a de novo occurrence with constitutional mosaicism (PS2_Very Strong; PMIDs: 26925222, 26475046). This variant is absent from gnomAD v2.1.1 and v3.1.1 (non-cancer) (PM2_Supporting). In vitro cleavage assays in HEK293 cells showed that this variant fails to produce 5p microRNAs from a pre-miRNA, indicating that this variant impacts protein function (PS3_Supporting, PMIDs: 22187960, 26545620). The computational predictor REVEL gives a score of 0.868, which is above the threshold of 0.75, evidence that correlates with impact to DICER1 function (PP3). This variant resides in the p.D1709 metal ion-binding residue located in the RNase IIIb domain of DICER1, that is defined as a mutational hotspot and critical functional domain by the ClinGen DICER1 VCEP (PM1, PMID: 31342592). In summary, this variant meets the criteria to be classified as PATHOGENIC for DICER1 syndrome based on the ACMG/AMP criteria applied, as specified by the ClinGen DICER1 VCEP: PS4_Moderate, PS2_Very Strong, PM2_Supporting, PS3_Supporting, PP3, PM1. (Bayesian Points: 15; VCEP specifications version 1; 02/11/2022)