MMP1 and glioblastoma: In addition, a soluble form of CD200 (sCD200), released following CD200 ectodomain shedding by matrix metalloproteinase-1 (MMP1) and ADAM19 metallopeptidase, is able to engage CD200R1, can be quantified in the serum, and has been correlated with worse tumor prognosis in CLL (25) and GBM patients (20).