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Alzheimer's disease (AD) and cancer are both age-related conditions, yet numerous large-scale epidemiological studies have consistently documented an inverse association, with individuals diagnosed with cancer exhibiting a reduced risk of AD and vice versa. Although this relationship has been replicated across diverse populations, its biological basis remains poorly understood. To address this gap, the present study applies a framework that integrates locus-level genetic correlation (rg) with genetically regulated gene expression to clarify the molecular factors contributing to the inverse epidemiological patterns observed between the two diseases. We used the largest available genome-wide association studies (GWAS) (Nmax = 448,150) to quantify local genetic correlations between AD and several age-associated cancers, including breast, prostate, lung, colorectal, melanoma, basal cell carcinoma, bladder, and endometrial cancer. Eight genomic regions showed significant negative local rg, at the 19q13.31-19q13.32 locus demonstrating strong negative correlations across multiple cancers, including breast, prostate, lung, melanoma, and endometrial cancer. To evaluate the contribution of genetically regulated gene expression, we conducted transcriptome-wide association studies (TWAS) using precomputed gene expression weights from cancer tissues (The Cancer Genome Atlas-TCGA), disease-agnostic tissues (Genotype-Tissue Expression-GTEx), and brain tissue (dorsolateral prefrontal cortex-DLPFC). For each AD-cancer pair, we prioritized genes that were nominally significant in both traits (p < 0.05) and exhibited inverse TWAS Z scores. This analysis identified 24 genes with opposite effect directions between AD and at least three cancer types. TWAS signals also aligned with local rg findings at the 19q13.31-19q13.32 region, suggesting that regulatory variation near this locus contributes to shared but opposing genetic effects beyond the canonical APOE signal. Across cancer types, genes inversely associated with AD converged on pathways involved in cell cycle regulation, apoptosis, DNA-damage response, and metabolic processes. These results support the hypothesis that biological mechanisms promoting proliferation and survival in cancer may oppose those contributing to neurodegeneration in AD.