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TP53 mutations represent one of the strongest adverse prognostic factors in myelodysplastic neoplasms (MDS). While multi-hit TP53 (TP53multiHit) alterations uniformly lead to very poor outcomes, the prognostic relevance of monoallelic TP53 (TP53mono) mutations remains controversial. TP53 variants can cause dominant-negative, loss-of-function, or gain-of-function effects. We hypothesized that functional heterogeneity among TP53 variants contributes to the variable clinical behavior observed in TP53mono-mutated MDS. Therefore, we analyzed pretreatment samples from 4,505 patients with MDS from two independent cohorts (IWG: n=3,173; J-MDS: n=1,332), including 271 patients with TP53mono and 499 with TP53multiHit. Functional annotation of TP53 variants was performed using a previously published phenotype score (PS) derived from saturation mutagenesis screens, capturing dominant-negative and loss-of-function effects. Median leukemia-free survival (LFS) differed significantly by TP53 allelic state (TP53 wild-type (TP53wt) 37.2 months; TP53mono 19.2 months; TP53multiHit 7.8 months; p<0.001). Within the TP53mono subgroup, functional annotation identified marked heterogeneity. Patients with high PS (≥7) showed a significantly inferior median LFS compared with those with low PS (12.6 vs. 43.3 months; p<0.001), particularly for IPSS-R and IPSS-M low-risk cases. Combining PS and variant allele frequency (VAF) further improved risk stratification. TP53mono patients with PS ≥7 and VAF ≥22% had a median LFS comparable to TP53multiHit cases (9.2 vs. 7.8 months, p=0.3), whereas those with PS <7 and VAF <22% exhibited a median LFS similar to TP53wt cases (44.7 vs. 37.2 months, p=0.8). Overall, functional annotation of TP53 variants refines prognostication in TP53mono-mutated MDS and may enhance individualized risk assessment.