Extended Data Fig. 2: The majority of thymocytes are actively cycling cells in both wild-type and Lbr^−/− mice.

Left, wild-type mice; right, Lbr^−/− mice. Thymus cryosections were immunostained with antibodies for Ki-67, a marker of cycling cells, and phosphorylated histone H3 S10 (H3S10ph), a marker for G2 and mitotic cells. In agreement with the idea that Lbr^−/− mice have a seemingly normal immune system⁴⁵, the number of cycling thymocytes in thymi of Lbr^−/− mice is comparable to that of wild-type mice. M, mitotic cells; G2, cells in mid/late G2. Ki-67 staining is shown as projections of 5-μm confocal stacks. Phosphorylated H3 S10 staining is shown as projections of 10-μm (for overviews) or 3-μm (for magnified areas) confocal stacks. Antibodies: mouse anti-phosphorylated H3 S10 (Abcam, ab14955) and rabbit anti-Ki-67 (Abcam, ab15580). Immunostaining and microscopy were performed as described in the Methods. Scale bars, 50 μm (top and middle) and 5 μm (bottom).