A family of plant integral membrane proteins, first discovered in barley. Mutants lacking wild-type Mlo proteins show broad spectrum resistance to the powdery mildew fungus, and dysregulated cell death control, with spontaneous cell death in response to developmental or abiotic stimuli. Thus wild-type Mlo proteins are thought to be inhibitors of cell death whose deficiency lowers the threshold required to trigger the cascade of events that result in plant cell death. Mlo proteins are localized in the plasma membrane and possess seven transmembrane regions; thus the Mlo family is the only major higher plant family to possess 7 transmembrane domains. It has been suggested that Mlo proteins function as G-protein coupled receptors in plants; however the molecular and biological functions of Mlo proteins remain to be fully determined.