Rap1 is required for the centralization of antigen-bound BCRs into a cSMAC. Primary B cells from MD4 mice, which express a transgenic BCR specific for HEL, were transduced with control siRNA or Rap1a/b siRNA. These cells were stained with Cell Trace Far Red (blue) and added to adherent Cos-7 cells expressing HEL-GFP (green). Timelapse spinning disk confocal microscopy was used to acquire images every 12 s starting at 1 min after adding B cells to the APCs and continuing until t = 12 min. The video is played back at 10 frames per second (120X real time).