ML133 hydrochloride is a potent and selective Kir2.1 inward rectifier potassium channel inhibitor, exhibiting pH-dependent IC50 values of 1.8 µM at pH 7.4 and 290 nM at pH 8.5. It shows minimal activity on Kir4.1 and Kir7.1 and no effect on Kir1.1, making it a valuable research tool for studying Kir2.1 function in cardiac electrophysiology and neuronal excitability in vitro.