H-L-Photo-leucine HCl is a diazirine-containing leucine amino acid and multifunctional photo-crosslinker. Its incorporation into peptides or small-molecule probes and tools allows for photoaffinity labeling of cellular targets and protein-protein interactions upon UV light (∼360 nm) irradiation to form a covalent bond. This and other multifunctional probe building blocks will continue to accelerate drug discovery research for probing cellular mechanisms, target ID/validation, and understanding traditionally undruggable targets. An Fmoc-protected version is also available as 907391.