We have studied the pattern of expression of the Q10 gene, a H-2 class-I gene located in the major histocompatibility complex which encodes a soluble class-I molecule, in the mid-gestation mouse embryo, and compared it to those of two other class-I genes, namely Kd and 37, the latter gene located in the thymus leukemia region. We found that the steady-state amount of these different mRNAs gradually increased from day 13 to day 18. By comparison with the level of expression of these genes in adult liver, the increase during gestation was fairly more marked for Q10 mRNA than for the others. Furthermore, we found that the Q10 gene is transiently expressed in the endoderm layer of the visceral yolk sac and in the fetal heart. Expression in the latter tissue decreases abruptly while increasing in the liver. It has been proposed that the Q10 protein is involved in immune tolerance. However, the time course of expression of Q10 mRNA and its tissue distribution during embryogenesis suggest that the Q10 protein could play a role in the differentiation of hematopoietic stem cells.