In this image of the corticospinal tract of a mouse are oligodendrocyte lineage cells (green) expressing a toxin that blocks proteins involved in vesicular trafficking and secretion. This manipulation causes oligodendrocyte maturation to stall at the NKX2.2 (blue)-negative premyelinating stage and results in accumulation of a kinase called Fyn in the cell somas (magenta). Stalled oligodendrocyte maturation ultimately culminates in impaired myelination and defects in spinal cord maturation. For more information, see the article by Fekete et al. (pages 6592–6608). Cover image: Christopher Fekete and Akiko Nishiyama.