To determine whether sCD200 released from CLL and Hek-hCD200 cells was capable of binding and phosphorylating CD200R1 on Hek293-CD200R cells, we probed for the presence of CD200R1 in protein immunoprecipitated with an anti-phospho-tyrosine antibody specific for the phosphorylated tyrosine in the CD200R1 tail (see Materials and Methods) following incubation of the cells with sCD200-containing supernatants. Here, CD200R1 is linked to B-cell chronic lymphocytic leukemia.