1. An elderly woman with a history of multiple oral ulcers presents with flaccid bullae on her scalp, face, and trunk. Manual pressure on the skin produces separation of the upper layer of the epidermis, followed by eventual sloughing of the skin. The patient has been in relatively good health until recently, and denies taking any medications. A biopsy of one of the skin lesions reveals separation of epithelial cells above the basal layer. Autoantibodies to which of the following components would most likely be found in this patient?