Two chloroplast-encoded proteins of photosystem II (PSII) have recently been the object of considerable interest. One is a rapidly synthesized 32-kD protein (Bottomley et al. 1974; Eaglesham and Ellis 1974; Hottman-Falk et al. 1982), also identified as a herbicide-binding protein (Pfister et al. 1981) and designated the QB protein (Hirschberg et al. 1984) because of its proposed role in a protein-quinone complex, which serves as the second stable electron acceptor of PSII (Arntzen et al. 1982). The other is a protein of similar size, called D2, that has several regions of homology with the QB protein (Rochaix et al. 1984). In this paper, we briefly review what is known about the primary structure of these two proteins and their genes and the mutations in the gene for the QB protein that affect herbicide resistance and cross-resistance.