Salicylaldehyde probe 3 reveals PF-06873600 target engagement in vivo. (a) Mice were dosed orally with vehicle (n=5) or PF-06873600 (50 mg/kg, n=5), and all mice were co-dosed with probe 3 (10 mg/kg) by subcutaneous injection. After 1 h, spleens were collected for chemoproteomic analysis. (b) Chemical structure of PF-06873600. (c) Volcano plot showing log2 fold-change and significance (−log10 p-value) between proteins enriched by probe 3 from mice treated with PF-06873600 vs. vehicle. Red, kinases; gray, non-kinases.