By use of in situ hybridization experiments, the mouse homeobox genes Hox-3.1 and Hox-1.5 are compared in the temporal and spatial patterns of their embryonic transcripts. Transcripts of both genes are first detected at about 7 1/2 days, although the appearance of Hox-3.1 transcripts apparently follows Hox-1.5 after a small delay. Hox-3.1 and Hox-1.5 transcripts occupy domains which are different, although overlapping, along the anteroposterior axis of the embryo. The domains are first established within the ectoderm and mesoderm germ layers at 7 1/2–8 days, but subsequently they persist within the nervous system, the prevertebral column and within at least some of the organs (the thyroid, lung, stomach, mesonephric and metanephric kidneys) at 12 1/2 days. In discussion, two different mechanisms are considered by which positional information might first be generated within the germ layers.

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