Micro Four Thirds mount. The E-P1 is the first Olympus model to feature the new Micro Four Thirds variant of the Four Thirds system. Micro Four Thirds does away with the mirror and allows for smaller, thinner cameras and lenses.
As there is no mirror and the camera works in live view mode full time, the E-P1, like the Panasonic G1 and GH1, features a newly-designed vertically traveling focal plane shutter that stays open when powered down (conventional shutters are closed when powered down). This makes MFT cameras rather unusual in exposing their sensors when the lens is removed.