Day 18 has 192 protein-coding genes. Over 50 of them encode proteins that build the epidermis, or the outer layer of the skin. This cluster of genes is called the epidermal differentiation cluster. Your skin is constantly being replaced by new cells that migrate from lower layers of the skin into the epidermis. Over the course of four to six weeks, they turn on these differentiation genes to secrete waterproof and flexible proteins into the epidermis, and undergo a kind of programmed death called cornification.
This cluster of genes is conserved in the same order (synteny) across mammals and reptiles. Their evolution was an important step in adapting to life out of the oceans.
Click here to see the LOR (loricrin) gene and Day 18. Note that almost half of the amino acids are glycine (colored gray), which is important to forming waterproof bonds between proteins.