The 563C>T variant in ITGB3 is a missense variant predicted to cause substitution of Serine by Leucine at amino acid 188 (p.Ser188Leu). Surface expression of αIIbβ3 measured by Western blot in COS-7 cells transiently co-transfected with Ser188 β3 (wild type) and Leu188 β3 (mutant) showed absent immunoprecipitation, indicating that this variant impacts protein function (PMID: 9684783)(PS3). At least one patient (Patient BL in PMID: 9684783) with this variant displayed mucocutaneous bleeding and impaired aggregation with all agonists except ristocetin, which is highly specific for Glanzmann thrombasthenia (PP4_Moderate). The GT patient from PMID: 9684783 was homozygous for this variant (0.5 PM3 points, PM3_Supporting). The computational predictor REVEL gives a score of 0.771, which is above the ClinGen PD VCEP threshold of >0.7 and predicts a damaging effect on function (PP3). The highest population minor allele frequency in gnomAD v4.1.0 is 0.000001695 (2/1180026 alleles) in the European (Non-Finnish) population, which is lower than the ClinGen PD VCEP threshold (<0.0001; PM2_Supporting). In summary, this variant meets the criteria to be classified as Likely Pathogenic for autosomal recessive Glanzmann Thrombasthenia based on the ACMG/AMP criteria applied, as specified by the ClinGen PD VCEP: PS3, PP3, PP4_Moderate, PM2_Supporting and PM3_Supporting (VCEP specifications version 2; date of approval 06/06/2024).