In situ interaction determination

Methods, reagents, compositions, and kits for in situ interaction determination (ISID) via interaction-dependent polymerase chain reaction (ID-PCR) are provided herein. ISID technology is useful for rapidly evaluating potential small molecule-target interactions from mixtures in a single solution. ISID is compatible with unpurified targets in biological samples and can be used to evaluate ligand-binding in DNA-encoded chemical libraries in cell lysates. ISID is also useful to screen ligand interactions of proteins or other molecules in their native state, including their native post-translational modifications and any interactions with accessory proteins and metabolites, in ways that better reflect their relevant biological environment. Because ISID is compatible with crude cell lysates, difficult-to-purify, poorly soluble, intrinsically unstable, and aggregation-prone targets may also be compatible with this method, without requiring truncation or other strategies used to promote heterologous expression.