STIM1 and clostridium difficile infection: Within Orai1 itself, structure-function studies have implicated the N-terminus (Bergsmann et al., 2011; Mullins et al., 2016b; Zhang et al., 2019), the C-terminus (Lee et al., 2009) as well as the intracellular TM2-TM3 loop (Srikanth et al., 2010) regions in CDI, but the identity of the Ca2+ sensor, the location of the inactivation gate, and the precise role of STIM1 for CDI, including the possibility that it serves as the enigmatic Ca2+ sensor, is poorly understood.