Anthropic found the one nucleotide that decides whether an antibiotic cures an infection or takes a patient's hearing. It was never asked to find a drug. It was asked why drugs built for bacteria keep hitting the ribosomes inside human cells. > PAIR - every bacterial drug pocket matched to its human twin and compared residue by residue > FLAG - any drug whose contacts survive in both pockets marked as a crossover risk > VARIANT - common human gene variants rebuilt into the pocket, one at a time > RANK - crossover risks sorted by how widely the drug is still prescribed today Most pockets were safely different. Evolution split the two ribosomes long ago, and most drugs only fit one of them. The danger sat in a small group where the gap is a single base. One nucleotide separates a cure from a side effect. In carriers of certain variants, that gap closes. The human pocket starts looking bacterial, and the drug cannot tell the difference. That is the known mechanism behind aminoglycoside hearing loss. The pass surfaced other drug classes sitting close to the same edge. None of this is a clinical finding yet. It is a ranked map of where to look, the kind a lab cannot afford to build by hand. Toxicity usually shows up in patients, years after approval. This checks the geometry before the first dose is ever given. Every paired pocket and the full crossover ranking are broken down underneath.