The diversity of protein isoforms is often generated from single genes by alternative splicing of the primary transcript. Using transfection of beta tropomyosin minigene constructs into homologous and heterologous cell systems, we show that there are differences, among higher vertebrates, in the components of the splicing machinery which control the conserved regulated splicing pattern of two mutually exclusive exons (6A and 6B) present in this gene. These experiments demonstrate that genes which give rise to alternative transcripts may require an appropriate combination of splicing factors which are species-specific, or at least restricted to the same taxonomic subgroup (class). An important practical implication is that the splicing of these genes may be deregulated in heterologous systems in vitro and in vivo, i.e. in transgenic animals.