Antibodies against contactin associated protein-like 2 (Caspr2) are found in some patients with autoimmune encephalitis. To see how these antibodies interact with Caspr2, cultured rat hippocampal neurons were stained with antibodies against the Caspr2 protein on the cell surface (red) and with antibodies against the Caspr2 protein that can penetrate the cell (green). Nearly all of the stained protein is golden colored (overlap of red and green), indicating that it was stained with both antibodies because it was on the surface of the cell. Caspr2 antibodies may cause encephalitis by binding to nerve cells in the brain and blocking their cell surface interactions with other cells. See Patterson et al., pages 40–51, this issue.