This study explores the mechanisms of action and resistance of the EZH1-EZH2 dual inhibitor valemetostat in the treatment of adult T-cell leukemia/lymphoma (ATL). Valemetostat reduces tumor size and shows durable clinical responses in aggressive lymphomas with multiple genetic mutations. Integrative single-cell analyses reveal that valemetostat disrupts the highly condensed chromatin structure formed by H3K27me3, neutralizing multiple gene loci, including tumor suppressor genes. However, long-term treatment leads to the emergence of resistant clones with reconstituted chromatin resembling the pre-treatment state. Acquired mutations at the PRC2 interface increase H3K27me3 expression, while mutations like TET2 or elevated DNMT3A expression cause chromatin recondensation through de novo DNA methylation. Subpopulations with distinct metabolic and gene translation characteristics are implicated in primary susceptibility until heritable mutations occur. Targeting epigenetic drivers and chromatin homeostasis may enhance sustained epigenetic cancer therapies. H3K27me3 is a cancer hallmark that accumulates around genes that should be properly expressed, leading to chromatin condensation and suppression of essential genes. EZH1 and EZH2 are compensatory factors that regulate methylation patterns. EZH2 is frequently abnormal in cancers and alters the epigenome by increasing H3K27me3 levels. Valemetostat, a first-in-class EZH1-EZH2 inhibitor, is effective against lymphomas and has shown sustained safety and efficacy in clinical trials for ATL and other lymphomas. Next-generation epigenetic therapies targeting H3K27me3 are promising, but the mechanisms of systemic therapies affecting the tumor epigenome remain unclear. Clinical trials show that valemetostat reduces abnormal lymphocytes and proviral load, with durable responses in patients. Chromatin decondensation by valemetostat restores the epigenome to a healthy state, leading to clinical improvements. Chromatin reprogramming by valemetostat is associated with the emergence of resistant clones due to PRC2 mutations or other epigenetic changes. Acquired PRC2 mutations or epigenetic factors like TET2 and DNMT3A can lead to resistance by recondensing chromatin and silencing tumor suppressor genes. DNA methylation plays a role in resistance, with increased mCpG near the TSS contributing to chromatin recondensation. DNMT3A and TET2 are involved in acquired resistance, with DNMT3A causing hypermethylation and TET2 loss leading to resistance. Subpopulations with differential susceptibility show distinct metabolic and transcriptional features, with SC-B being the origin of resistant clones. These findings highlight the importance of chromatin structure in cancer progression and the potential of epigenetic therapies for sustained treatment.This study explores the mechanisms of action and resistance of the EZH1-EZH2 dual inhibitor valemetostat in the treatment of adult T-cell leukemia/lymphoma (ATL). Valemetostat reduces tumor size and shows durable clinical responses in aggressive lymphomas with multiple genetic mutations. Integrative single-cell analyses reveal that valemetostat disrupts the highly condensed chromatin structure formed by H3K27me3, neutralizing multiple gene loci, including tumor suppressor genes. However, long-term treatment leads to the emergence of resistant clones with reconstituted chromatin resembling the pre-treatment state. Acquired mutations at the PRC2 interface increase H3K27me3 expression, while mutations like TET2 or elevated DNMT3A expression cause chromatin recondensation through de novo DNA methylation. Subpopulations with distinct metabolic and gene translation characteristics are implicated in primary susceptibility until heritable mutations occur. Targeting epigenetic drivers and chromatin homeostasis may enhance sustained epigenetic cancer therapies. H3K27me3 is a cancer hallmark that accumulates around genes that should be properly expressed, leading to chromatin condensation and suppression of essential genes. EZH1 and EZH2 are compensatory factors that regulate methylation patterns. EZH2 is frequently abnormal in cancers and alters the epigenome by increasing H3K27me3 levels. Valemetostat, a first-in-class EZH1-EZH2 inhibitor, is effective against lymphomas and has shown sustained safety and efficacy in clinical trials for ATL and other lymphomas. Next-generation epigenetic therapies targeting H3K27me3 are promising, but the mechanisms of systemic therapies affecting the tumor epigenome remain unclear. Clinical trials show that valemetostat reduces abnormal lymphocytes and proviral load, with durable responses in patients. Chromatin decondensation by valemetostat restores the epigenome to a healthy state, leading to clinical improvements. Chromatin reprogramming by valemetostat is associated with the emergence of resistant clones due to PRC2 mutations or other epigenetic changes. Acquired PRC2 mutations or epigenetic factors like TET2 and DNMT3A can lead to resistance by recondensing chromatin and silencing tumor suppressor genes. DNA methylation plays a role in resistance, with increased mCpG near the TSS contributing to chromatin recondensation. DNMT3A and TET2 are involved in acquired resistance, with DNMT3A causing hypermethylation and TET2 loss leading to resistance. Subpopulations with differential susceptibility show distinct metabolic and transcriptional features, with SC-B being the origin of resistant clones. These findings highlight the importance of chromatin structure in cancer progression and the potential of epigenetic therapies for sustained treatment.