Serum level of circulating CS1 is detected only in myeloma patients. (A) CS1 ELISA-positive (MM6) or -negative (MM9) serum samples were immunoprecipitated (IP) with isotype control, HuLuc63, or ChLuc90 mAb covalently attached to Dynal Tosylactivated Dynabeads, followed by immunoblotting (IB) using anti-CS1 mAbs ChLuc90 (left) or 1G9 (right). (B) CS1 ELISA was done in serum samples from MM patients (n = 52) and healthy donors (n = 34). P was calculated by χ² test from 2 × 2 contingency table (P < .001). N.D. indicates not detectable. (C) CS1 ELISA was performed in additional serum samples from newly diagnosed MM patients (n = 199). Patients with ISS I (n = 100) had significantly lower levels of CS1 than ISS II (n = 53) and III (n = 46; P = .006).