Protein phosphorylation controls many cellular processes. Although four amino acids—serine, threonine, tyrosine, and histidine—can be targets for phosphorylation, good reagents for routine detection have been available for only three of the four. The lack of suitable reagents for histidine phosphorylation can be blamed on the chemical instability of the modification and the failure to raise good antibodies against it. Tom W. Muir and coworkers at Princeton University have used a stable phosphohistidine analog, phosphoryl-triazolylethylamine, to produce antibodies that detect in vitro and in vivo histidine phosphorylation independent of the surrounding protein sequence (Nat. Chem. Biol. 2013, DOI: 10.1038/nchembio.1259). They successfully used the antibody in various biological assays, including enzyme-linked immunosorbent assays (ELISAs), western blots, and immunoprecipitation. A survey of histidine phosphorylation in Escherichia coli cell lysates, which was done in conjunction with mass spectrometric analysis, revealed that the amount of phosphorylation depends on the carbon source and the availability of nitrogen in the growth medium. One drawback of the new antibody is minor cross-reactivity with phosphorylated tyrosine.