Although most sexual dimorphism appears after gonadal differentiation, some occur at earlier stages of development. One example of this phenomenon is the skewing of the sex ratio of embryos due to the glucose concentration of the culture medium. This skewing occurs because male and female embryos differ in their abilities to metabolize glucose. Another example is the sexually dimorphic expression of the bovine embryo-derived signal for maternal recognition of pregnancy, IFNT. The difference in expression of X-linked genes between male and female embryos, that results from incomplete X-chromosome inactivation, is considered to be involved, either directly or indirectly, in the sexual dimorphism that appears during early development.