(A) The relationship between the expression intensity (EST count) and breadth (number of tissues) of human and mouse genes. (A) The relationship between the expression intensity (EST count) and breadth (number of tissues) of human and mouse genes. Mouse genes with Precambrian (shaded bars) and later (open bars) origins were grouped on the basis of their level of expression in the mouse embryo. The average breadth of expression was computed for each group. The difference in the average breadth of expression between categories is significant at the 1% level using the Mann-Whitney U-test for the Precambrian genes. For the vertebrate-specific genes, the difference between the breadth of expression for genes belonging to high and medium expression levels is not significant at a 5% level. (B) The relationship of the level of expression (oligo-nucleotide array data) and protein divergence for 392 Precambrian human-mouse genes that are found to be expressed in all 45 mouse tissues. The differences in the average protein divergences between the genes belonging to high and medium or medium and low expression levels are significant at the 1% level using the Mann-Whitney U-test. Sankar Subramanian, and Sudhir Kumar Genetics 2004;168:373-381 Genetics Society of America