small molecule small molecule probe A small molecule useful to interrogate a biological system of interest. It has to be active in the system studies, i.e. cell permeable or active in vivo and be sufficiently efficacious and selective. The term probe also refers to the MLI screening effort, which has the goal to generate chemical probes. In addition, the MLI requires a novel chemical probe to be superior to the state of the art in terms of efficacy and selectivity and in-vivo properties. A list of probes generated is available here: http://mli.nih.gov/mli/mlp-probes/