The problem. Metal-tag imaging (imaging mass cytometry, MIBI) achieves high multiplexing but needs specialised, slow, destructive mass-spectrometry instruments. Most labs have a fluorescence microscope, not an ion beam. Could you reach comparable marker counts using standard optics, so highly-multiplexed tissue imaging isn’t gated on rare hardware?

The idea. CODEX (co-detection by indexing) tags each antibody with a unique DNA barcode. Rendering is iterative: fluorescent nucleotide analogues are polymerised to reveal a few barcodes at a time, imaged, stripped, and repeated over many cycles. The tissue stays intact on a normal microscope while dozens of proteins are read out in sequence, then computationally stacked into one high-dimensional image — here used to dissect the cellular architecture of the mouse spleen.

Why it matters. CODEX brought spatial proteomics within reach of ordinary labs, which is why it spread quickly and why the STU cares — accessibility decides which platform a unit can actually run. It completes the spatial-proteomics trio (imaging mass cytometry, MIBI, CODEX) that mirrors the RNA-imaging methods from the spatial day, and its neighbourhood analysis of the spleen prefigures the niche and domain questions the earlier reading raised.

Verdict. A widely-adopted, hardware-friendly multiplexed imaging method; cyclic imaging is time-consuming and demands careful antibody and registration workflows. Read it as spatial proteomics for the rest of us — many proteins, one microscope.