Volume 5 of Advances in Developmental Biology and Biochemistry consists of seven chapters that review specific aspects of development in several different organisms, including sea urchins, flies, worms, frogs, and mice. In Chapter 1, Wassarman and Therrier provide a detailed analysis of RAS1-mediated photoreceptor development in Drosophila as determined by genetic and biochemical approaches. Much of the information in this area has come from studies of the role of RAS1 in the Sevenless (SEV) signal transduction pathway that specifies the R7 photoreceptor cell fate. In Chapter 2, Westlund, Berry, and Schedl describe the regulation of germline proliferation in the nematode, C. elegans. In large part, the authors discuss components of signaling pathways, including glp-1 and other members of the Notch protein family that are crucial for a germ cell's decision of whether to remain mitotic or to enter meiotic prophase. In Chapter 3, Richards details the current status of the ecdysone regulatory cascade in Drosophila. In a comprehensive presentation, he describes ecdysone-regulated chromosomal puffs, their corresponding genes, and the receptors and transcription factors involved in the ecdysone regulatory cascade. In Chapter 4, Tam, Quinlan, and Trainor describe the patterning of progenitor tissues for the cranial region of the mouse embryo during gastrulation and early organogenesis. In particular, the authors review the sequence of developmental processes that delineate major tissue domains and the finer segmental organization of the progenitor tissues for the formation of craniofacial structures. In Chapter 5, Bodmer and his colleagues detail the genetic basis of heart development in Drosophila. Here, the authors discuss various gene functions that are required for the initial specification and later differentiation of the heart and compare molecular mechanisms that lead to heart formation in Drosophila with those that may be involved in heart formation in vertebrates. In Chapter 6, Tata Discusses hormonal signaling and amphibian metamorphosis. Following a review of amphibian metamorphosis, the author describes thyroid hormone receptors, the genes encoding them, and their target genes, as well as how their activities are coordinated by signaling molecules. Finally, in Chapter 7, Kaldis and colleagues discuss the functions of creatine kinase (CK) isozymes in spermatozoa from a variety of mammalian and nonmammalian species.