5-Hydroxymethylcytosine: a key epigenetic mark in cancer and chemotherapy response.

5-hydroxymethylcytosine (5hmC), an epigenetic modification derived from the oxidation of 5-methylcytosine (5mC) by the ten-eleven translocation (TET) family of dioxygenases, plays a pivotal role in the regulation of gene expression, cellular differentiation, and developmental plasticity. Once considered an intermediate in DNA demethylation, 5hmC is now recognized as a stable and functionally significant epigenetic mark with distinct genomic distributions and significant regulatory implications. This review provides a comprehensive analysis of the biological functions of 5hmC in normal cellular processes, including its role in maintaining tissue-specific gene expression, lineage commitment, and genomic integrity. We also describe its role in cancer, the mechanistic underpinnings of its loss or redistribution in tumor cells, and how these changes contribute to oncogenic signaling pathways, epithelial-mesenchymal transition, and tumor heterogeneity. Furthermore, we explore the utility of 5hmC as a biomarker in cancer diagnostics and prognostics, supported by recent advances in sequencing technologies and cell-free DNA profiling. We also examine the intersection of 5hmC and chemotherapy, highlighting how aberrant 5hmC levels can influence drug resistance and sensitivity, and assess the therapeutic potential of targeting TET enzymes and associated pathways. By integrating insights from basic epigenetics, cancer biology, and therapeutic research, this review underscores the multifaceted role of 5hmC in human malignancies and outlines the translational opportunities for exploiting 5hmC-related mechanisms in precision oncology.