Cancer remains a leading cause of death worldwide, yet current diagnostic tools fall short in sensitivity, comprehensiveness, and cost-effectiveness, especially when linking rare somatic mutations to functional consequences across individual cells. To address this, we developed MultiCloneSeq, a novel single-cell multi-omics technology that simultaneously profiles the full-length transcriptome and epigenome of individual cancer clones. MultiCloneSeq overcomes key limitations of current tools (e.g. 10x Genomics) by enabling full-length RNA capture, detecting non-polyadenylated transcripts, and improving sensitivity through clone-based amplification using microfluidics. It integrates three proprietary methods: CloneSeq for single-clone culture and sequencing, scrDSeq for full-length total RNA profiling, and a cost-effective drop-based assay for chromatin accessibility. Applied to lung carcinoma and soft-tissue sarcoma, MultiCloneSeq identified rare subpopulations, such as cancer stem-like cells and clinically relevant mutations (e.g. in EGFR), undetectable by standard methods. Compared to existing platforms, it reduces cost by 10-fold, paving the way for practical, high-resolution diagnostics. Originating from our ERC-funded SC-EpiCode project, this innovation addresses cellular heterogeneity by decoding gene expression and chromatin regulation. The PoC phase will expand patient sample cohorts, strengthen IP, and lay the groundwork for commercialization as a precision oncology tool for early cancer detection and personalized treatment strategies.