CD74 downstream signaling cascade leads to B-cell proliferation and entry to S phase. (A-C) IgD⁺ B cells from control and B cells from CD74^–/– mice were stimulated with anti-CD74 or irrelevant anti-CD8 antibodies for 24 hours. (A) BrdU incorporation was followed as described in “Materials and methods.” The results presented are representative of 5 independent experiments. (B-C) [3H] thymidine incorporation was followed as described in “Materials and methods.” The results shown for 24 hours (B) or 48 hours (C) represent the average of at least 4 independent experiments with similar results. The assay was performed in triplicate, and the results presented show the average of each treatment. Error bars represent the standard deviation of the results in the triplicates of each treatment. (D) PI staining was performed as described in “Materials and methods.” The results presented are representative of 3 different experiments.