We have found a novel gene (alien) that is expressed exclusively in the muscle attachment sites (apodemes) during embryogenesis in Drosophila. Antibodies raised against the Alien protein enable us to follow the developing attachments from state 11/12 until stage 16/17. The coding region of the Drosophila alien gene is highly conserved to a gene of unknown function, isolated from a plant (Loo et at., 1995), and to the human TRIP15 gene (Lee et al., 1995). Searching for thyroid receptor interacting proteins, TRIP15 was isolated as a negative regulator. Whether there is a functional correlation to Alien remains to be analyzed. Alien expression is independent of muscle formation, as shown in rolling stone mutant embryos. Even in twist and snail mutants, lacking mesodermal development, alien expression is fairly normal, showing a rather autonomous development of the apodemes. The conservation of alien suggests an important role in differentiation.