Mutually exclusive expression of two dominant-negative helix-loop-helix (dnHLH) genes, Id4 and Id3, in the developing brain of the mouse suggests distinct regulatory roles of these dnHLH proteins during cellular proliferation and differentiation of the nervous system.

The dominant-negative helix-loop-helix (dnHLH) proteins Id1 and Id2 have been implicated in the regulation of cell proliferation and differentiation in myogenesis, neurogenesis, and/or hematopoiesis. To further investigate the functional role of dnHLH proteins, we have performed in situ hybridization analysis on serial sections of mouse embryos from days 9.5 to 17.5 postcoitus to establish the spatial and temporal expression patterns of Id3 (HLH462) and Id4, a recently isolated fourth member of the mammalian dnHLH gene family. Id3 transcripts are present throughout embryogenesis and are found in neural cells as well as in cartilage primordia and in epithelial cells lining a variety of organs. The spatial expression pattern of Id3 overlaps considerably with the previously determined pattern of Id1. Id4 expression, which is up-regulated during embryogenesis, is restricted to specific cells of the central and peripheral nervous system. Within the detection limits of in situ hybridization, Id4 and Id3 expression is mutually exclusive in neural precursor cells of the developing brain, suggesting distinct regulatory functions for these dnHLH proteins during neurogenesis.