Circular transcripts generated by activated B cells. (A) Circular transcripts detected after LPS plus cytokine stimulation. Iγ2b and Iγ3 were analyzed after a 36-hour stimulation; Iγ2a and Iα after 3 days. (B) Iγ2a and Iα production after 3 days of stimulation with anti-CD40 plus cytokine. RT-PCR products were amplified from 1μg total RNA extracted from stimulated cells. The same patterns were observed in at least 3 independent experiments. LB, large activated B cells isolated by centrifugation on the Percoll gradient; SB, small resting B cells purified by Percoll gradient; the remaining samples were from small resting B cells stimulated in vitro. W, wild-type B cells; M, Sap-deficient B cells. The transcript of the mouse β-actin gene was used as the amplification control.