It is unknown whether cross-regulatory interactions between homeotic genes, which have been shown to play an important role in the maintenance of their expression domains during Drosophila development, are also important during mammalian development. We have analyzed here the expression of Hox genes in Hoxa-1 (Hox-1.6) null mutant embryos to investigate the possible existence of regulatory interactions between Hoxa-1 and other Hox genes. We show that the absence of a functional Hoxa-1 gene product does not globally interfere with the expression of other Hox genes in terms of both spatial boundaries and transcript abundance. However, a limited area of the hindbrain shows a strong reduction in Hoxb-1 (Hox-2.9) and Krox-20 transcripts, which most likely reflects a marked reduction in size of the former fourth and fifth rhombomeres. These alterations coincide with the region that is subsequently affected in Hoxa-1 null mutant mice and suggest that the primary defects in this mutation are spatially restricted deletions of some rhombomeric structures.