More recently, using a Cd200 conditional null mouse model, we identified the collagen peptidase Cathepsin K (Ctsk) as a crucial target gene of the Cd200-Cd200r signaling axis in Cd200r+ tumor-infiltrating myeloid lineages, which was required for tumor cell invasion and metastasis [54] (Figure 1B). This evidence concerns the gene CTSK and neoplasm.