In general, it is difficult to identify genes that play critical roles in developmental decisions in vertebrates because it is not possible to perform exhaustive screens for mutations that affect developmental processes. While some genes have been identified and analysed by conventional genetic methods, for example the mouse T locus (Bennett, 1975) or the axolotl o mutation (Brothers, 1976), it is likely that many genes that control vertebrate development will have to be identified by other means. In recent years, two methods for identifying such genes have been utilized. First, genes have been selected for study because they are expressed at a particular time or place during embryogenesis. For example, genes that are expressed only during gastrulation (Sargent & Dawid, 1983; Krieg & Melton, 1985) or at the cellular blastoderm stage (Roark, Mahoney, Graham & Lengyel, 1985) have been cloned and there are numerous examples of genes being studied because they are expressed in some, but not all tissues (e.g. Lynn et al. 1983) or are spatially localized within single cells (Jeffery, Tomlinson & Brodeurer, 1983; King & Barklis, 1985; Rebagliati, Weeks, Harvey & Melton, 1985).

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