Day 14 has 67 protein-coding genes, including PTPN22 (Protein tyrosine phosphatase, non-receptor type 22 lymphoid). The PTPN22 protein is expressed in cells of the immune system, and is one small part of the complicated signaling cascades that they use to coordinate their activity.
PTPN22 is a particularly interesting gene because variation in the gene is associated with many autoimmune disorders like rheumatoid arthritis, Crohn’s disease, lupus, and vitiligo. One variant, rs2476601 (R620W) is present in about 20% of people with European ancestry, and increases the risk of autoimmune diseases by about twofold. Why wouldn’t evolution rid the population of this mutation? It may confer some benefit by strengthening the immune response to infection, at the same time it increases the chance the immune system will attack the body.
The PTPN22 gene is shared across four-legged vertebrates, meaning it is at least 360 million years old.
Click here to see the rs2476601 mutation in PTPN22 (flashing).