NM_006758.2:c.101C>T (p.Ser34Phe) in U2AF1 is classified as Likely Pathogenic using generic ACMG/AMP 2015 criteria (PMID:25741868) with 1 strong, 1 moderate, and 3 supporting pathogenic criteria.¹ Well-established functional studies demonstrate that the U2AF1 S34F substitution has a damaging gain-of-function effect: it alters 3′ splice site recognition, changes sequence specificity of pre-mRNA binding, promotes aberrant exon skipping, induces R-loop accumulation and ATR checkpoint activation, and produces altered hematopoiesis in transgenic mouse models (PS3_Strong).² The variant is located at codon 34 in the first CCCH zinc finger domain, a well-established mutational hotspot where recurrent somatic missense mutations (S34F, S34Y) are observed in myelodysplastic syndromes and acute myeloid leukemia (PM1_Moderate).³ The variant is extremely rare in population databases: gnomAD v2.1 reports 1 allele in 281,256 (AF=3.56×10⁻⁶) and gnomAD v4.1 reports 0 alleles in 18,936, both well below the PM2 threshold of 0.1% (PM2_Supporting).⁴ U2AF1 has a low rate of benign missense variation and missense variants at the zinc finger domains are the established disease mechanism, supporting PP2 at a supporting level (PP2_Supporting).⁵ Multiple in silico predictors support a deleterious effect: REVEL score of 0.7 is above the pathogenic threshold; BayesDel (0.415) is borderline (PP3_Supporting).⁶ No benign criteria were met. The variant is not a common polymorphism (BA1/BS1 not met), functional studies show a damaging effect (BS3 not met), and computational evidence does not support a benign interpretation (BP4 not met).⁷ Limitations: all functional evidence is derived from somatic disease contexts (MDS/AML cell lines and patient samples); no germline-specific functional data, de novo reports, cosegregation data, or germline case-control data are available. PVS1, PS1, PS2, PS4, PS5, PM5, PM6, PP1, PP4, PP5, and most benign criteria were not assessed due to absent or insufficient evidence.