Nuclear receptors are ligand-inducible transcription factors which respond to endocrine, paracrine and, possibly, autocrine signals, thus triggering key events in early embryonal development, differentiation and homeostasis. Here we discuss recent results which may lead to an understanding of the molecular mechanisms of transcriptional activation by a given transcription activation function (TAF) of these activators and of the receptor isoform-specific activation of transcription. In addition, we present a concept of the mechanism of action of two different types of antihormones.