Mesoderm formation is the first major differentiative event in vertebrate development. Many new mesoderm-specific genes have recently been described in the mouse, chick, frog and fish and belong to classes comprising T-domain genes, homeobox genes and those encoding secreted proteins. The T-domain genes have different but overlapping expression patterns and, in Xenopus, can ectopically activate nearly all other mesodermal genes. Several new homebox genes seem to mediate the ventralising activity of bone morphogenetic protein. New genes encoding secreted proteins induce dorsal mesoderm, in some cases by antagonizing ventralising factors.