Cross-linked envelope protein of keratinocytes. It is a keratinocyte protein that first appears in the cell cytosol, but ultimately becomes cross-linked to membrane proteins by transglutaminase. All that results in the formation of an insoluble envelope beneath the plasma membrane (By similarity). Expressed in uterus. Expression in uterus varies during the estrous cycle, with highest levels during proestrus and estrus stages and declining sharply from metestrus to diestrus. During early pregnancy, uterine expression is markedly increased at 1 dpc, declines rapidly at 2 dpc and is almost undetectable from 3 dpc to 6 dpc. Up-regulated by estrogen in the uterus of ovariectomized animals, with strongly increased expression detected in luminal epithelial and stromal cells at 6 and 12 hours after hormone injection. Belongs to the cornifin (SPRR) family.