CD137 expression increases HRS cell proliferation. Indicated numbers of L-428 (A) or L-1236 (B) or KM-H2 (C) cells were seeded (white bar: non-CD137-expressing cells, black bar: CD137-expressing cells), and pulsed with 0.5 μCi of 3H-thymidine at the same time. The cells were harvested 24 h later, and 3H-thymidine incorporation was quantified. Depicted are means ± SD. *p <0.05, **p <0.01, ***p < 0.001.