The problem. Single-cell RNA-seq measures transcripts, but cell identity — especially for immune cells — is often defined by surface proteins, the markers flow cytometry has used for decades. Measuring both in the same cell used to mean separate assays on separate cells, losing the pairing that makes the comparison meaningful.

The idea. CITE-seq tags antibodies with DNA barcodes (“oligo-tagged antibodies”). Stain cells, run them through a standard droplet scRNA-seq platform, and the antibody barcodes get sequenced right alongside the mRNA — so each cell yields both its transcriptome and a readout of chosen surface proteins. It piggybacks on existing chemistry rather than inventing a new instrument.

Why it matters. This is an early, clean example of multimodal single-cell measurement — the direction the whole field, and the spatial/single-cell reading arc, keeps moving. It connects the transcriptomic view I know to the protein-level identity that clinicians and immunologists actually use. Methodologically it’s elegant: convert a protein measurement into a sequencing readout so one machine captures both.

Verdict. Foundational for multimodal single-cell; the antibody-barcoding idea recurs everywhere (cell hashing, feature barcoding). Read it for the design move — turning proteins into sequenceable tags — and as context for why modern single-cell analysis expects more than one modality per cell.