Phosphorylation of proteins on tyrosine is particularly important in the control of cell proliferation and differentiation, and drives many of the changes seen in cancer cells. Protein tyrosine phosphorylation has previously been thought to occur only inside cells, where it can control changes in cell structure, movement, and gene expression. In a recent article in Cell, the Whitman Lab found a new kind of tyrosine kinase, VLK, that is secreted into the extracellular environment. VLK phosphorylates a broad range of extracellular proteins, including key regulators of tumor invasion and metastasis, blood clotting, and inflammation, pointing to a new mechanism for the control of protein function outside of cells.