Single-cell splice isoform usage reveals distinct axes of cellular identity and senescence.

Alternative splicing greatly expands the diversity of gene products encoded by the human genome. Single-cell transcriptomic atlases have characterized human cell types through gene-level expression, but short-read sequencing has limited the ability to resolve full-length isoforms and their functional consequences. Here, we present a cross-tissue single-cell long-read isoform atlas spanning 26 human tissues. We identify hundreds of thousands of novel isoforms along with their cell-type-specific usage, and discover that over one-third of expressed isoforms are absent from existing reference databases. We further demonstrate that isoform usage is a structured, measurable axis of cellular identity that is distinct from gene expression. Applying this framework to cellular senescence, we resolve p16 INK4a and p14 ARF transcripts from the CDKN2A locus in individual cells and uncover cell-type-dependent isoform remodeling associated with the p16 INK4a senescence program. This isoform-resolved single-cell atlas offers a versatile framework to dissect the cellular logic of isoform regulation in senescence and beyond.