A protein in the cell nucleus that mediates the actions of retinoids (the natural and synthetic analogs of vitamin A) by regulating the rates of transcription of retinoid-responsive genes. Retinoid receptors facilitate the normal biological actions of vitamin A within the body. Thus, they play an important role in the maintenance of normal growth and development; in the immune response; in male and female reproduction; in blood cell development; and in the maintenance of healthy skin and bones; and ultimately in the general good health of the organism.