High mobility group (HMG) proteins are abundant components of mammalian nuclei and fall into three families. The members of one such family, HMG1 and HMG2, are ubiquitously expressed and facilitate the formation of nucleoprotein complexes where the DNA is sharply bent. We have identified a mouse cDNA that codes for a novel 200-amino-acid protein of the HMG1/2 family, which we called HMG4. The mouse Hmg4 gene is highly expressed in the embryo; Hmg4 transcripts are barely detectable in adult tissues. The human HMG4 gene, which is extremely similar to its mouse homolog, has been sequenced as part of chromosome X, band q28. HMG4, HMG1, and HMG2 proteins have been highly conserved during vertebrate evolution, suggesting that each has at least some unique property. It is possible that HMG4 is required during development.