Used primarily in biochemical research for probing the structure and function of proteins and enzymes. The mercury group binds selectively to sulfhydryl groups (-SH) in cysteine residues, making it useful for studying active sites in enzymes. It has been employed to inhibit specific enzymes reversibly, allowing researchers to examine reaction mechanisms and enzyme kinetics. Also utilized in early studies of membrane transport proteins to investigate thiol-dependent transport processes. Due to its reactivity and toxicity, it is handled with care and has largely been replaced by less hazardous thiol-reactive agents in modern applications.