The majority of probes have primary targets specified in their sources. In most cases these are supported by quantitative in vitro binding data (e.g. Ki, IC50 or Kd). Some may also have secondary targets with data-supported potency below that against the primary target (we have avoided using the term "offtarget" since there are few cases where secondary targets have been mechanistically assigned as a side-effect or toxicity liabilities). The 132 probes without primary target annotation were, in most cases, directed against viruses, bacteria, cell lines or pathways. They also predominantly belonged to the MLP and Nature Chemical Biology legacy sets. For the remainder, we collated 819 single and multi-component protein targets with 549 for both experimental and calculated with an overlap of 279. In total these constituted 807 distinct single protein identifiers (i.e. UniProt IDs 51 ), 544 for experimental and 535 for calculated probes with 272 in-common. The human Swiss-Prot target count was 796.