SIX5 belongs to a family of highly conserved homeodomain transcription factors implicated in development and disease [
1,
2 and
3]. The mammalian
SIX5/SIX4 gene pair is likely to be involved in the development of mesodermal structures [
4,
5 and
6]. Moreover, a variety of data have implicated human
SIX5 dysfunction as a contributor to myotonic dystrophy type 1 (DM1), a condition characterized by a number of pathologies including muscle defects and testicular atrophy [
7,
8 and
9]. However, this link remains controversial. Here, we investigate the
Drosophila gene,
D-Six4, which is the closest homolog to
SIX5 of the three
Drosophila Six family members [
10]. We show by mutant analysis that
D-Six4 is required for the normal development of muscle and the mesodermal component of the gonad. Moreover, adult males with defective
D-Six4 genes exhibit testicular reduction. We propose that D-Six4 directly or indirectly regulates genes involved in the cell recognition events required for myoblast fusion and the germline:soma interaction. While the exact phenotypic relationship between
D-Six4 and
SIX4/5 remains to be elucidated, the defects in
D-Six4 mutant flies suggest that human
SIX5 should be more strongly considered as being responsible for the muscle wasting and testicular atrophy phenotypes in DM1.