Tag:D4H corresponds to the cholesterol-binding D4 domain of perfringolysin O theta-toxin, with a D434S amino acid substitution that lowers the threshold for binding to cholesterol. Tag:D4H can be used to monitor the subcellular distribution of cholesterol, if fused to an appropriate reporter protein, or to target a tagged protein to a particular subcellular localization via cholesterol binding (PMID:25663704).