ABSTRACT
We showed earlier (Gaunt, Miller, Powell & Duboule, 1986) that the mouse homoeobox gene Hox-1.5 is expressed in posterior ectoderm and mesoderm of 
and 
-day embryos, and in the 
-day nervous system posterior to a discrete boundary within the hindbrain. In further in situ hybridization experiments, it is now shown that restriction of Hox-1.5 expression to the posterior regions of the embryo can be detected at stages of development between 
and 
days. During this period, the intensity of transcription in presomitic and somitic mesoderm declines rélative to that in the overlying neural ectoderm, and the transcription boundary within the presumptive hindbrain region sharpens. Hox-1.5 expression posterior to the hindbrain boundary is detected in the 
and 
-day embryo, but this is no longer found in newborn mice. Embryos of ages 
and 
days showed no evidence of Hox-1.5 transcripts. It is concluded that embryos undergoing gastrulation (at 
days) are the earliest stage at which Hox-1.5 transcripts can be detected by the in situ hybridization technique. In discussion, it is shown how this lies within the period of development during which tissues become determined along the anteroposterior axis of the mouse. Since there may be anterior-to-posterior variation in the time of determination along the body axis, it is suggested that homoeobox genes expressed more posteriorly, such as Hox-3 (Awgulewitsch et al. 1986), might start expression at times later in development.
