In 1980 Janet Rossant and William I. Frels published their paper, "Interspecific Chimeras in Mammals: Successful Production of Live Chimeras Between Mus musculus and Mus caroli," in Science. Their experiment involved the first successful creation of interspecific mammalian chimeras. Mammalian chimeras are valuable for studying early embryonic development. However, in earlier studies, clonal analysis was restricted by the lack of a cell marker, present at all times, that makes a distinction between the two parental cell types in situ. To battle this limitation, Rossant and Frels decided to make chimeras from embryos of two different species in order to have sufficient genetic differences so that, in any tissue type, the two cell types could be clearly identified. In their paper Rossant and Frels describe the successful creation of live chimeras between Mus musculus and Mus caroli. These two species of mice are more closely related than chimeras produced previously. The chimeras created in the experiment showed no sign of selection against one cell type or the other. Therefore, they are valuable for clonal analysis of development. Rossant and Frels were the first to successfully produce an interspecific mammalian chimera that experienced normal development.