Expression of the rat, S-100-related, calcium-binding protein gene, p9Ka, in transgenic mice demonstrates different patterns of expression between these two species.

p9Ka (also known as mts1/18A2/calvasculin/CAPL) is a member of the S-100-related family of small, calcium-binding proteins. Previous studies suggest apparent discrepancies between the expression of the p9Ka gene in rat, mouse, and human tissues. Here we demonstrate that the natural p9Ka gene is expressed at lower levels in mouse than in rat, and that, in mouse but not in rat, p9Ka mRNA is more highly expressed in cells of lymphoid origin. Transgenic mouse strains express rat-p9Ka transgenes in a gene copy-number-dependent manner. The rat p9Ka transgene mRNA shows the same tissue distribution in several lines of transgenic mice, a distribution that is characteristic of the rat, from which the transgenes were derived. These results show that there is a difference in the pattern of expression of the same gene in two closely related species, and that the pattern of expression found in rat is specified by the DNA in the rat gene itself.