Phototropisms of plant organs such as hypocotyls, coleoptiles, and roots are induced by unilateral blue light. These phototropisms result from differential growth, thought to be mediated by asymmetrical distribution of auxin (see Chapter 31 by Iino and Haga). Phototropins 1 and 2 (phot1 and phot2)have been identified as the photoreceptors involved in phototropism of Arabidopsis thaliana (see Chapter 15 by Briggs and Chapter 31 by Iino and Haga, respec- tively). Mutants of Arabidopsis impaired in phototropism, including nph2, nph3, nph4, and rpt2, have been analyzed. These studies are providing valuable infor- mation on the signaling pathway of phototropism (Liscum 2002). NPH3 and RPT2 have been identified as signal transducers functioning specifically for pho- totropism. NPH4 has been identified as a transcriptional regulator involved in the control of phototropic and gravitropic differential growth. NPH2 has not yet been analyzed in detail. This article reviews the functions and structural proper- ties of NPH3 and RPT2.