S100A8 and S100A9 (also known as MRP8 and MRP14, respectively) are Ca2 binding proteins belonging to the S100 family. They often exist in the form of heterodimer, while homodimer exists very little because of the stability. S100A8/A9 is constitutively expressed in neutrophils and monocytes as a Ca2 sensor, participating in cytoskeleton rearrangement and arachidonic acid metabolism.

> 95% as determined by Tris-Bis PAGE