TBL1XR1 and microcephaly: Conversely, the decreased MAPK signaling in Tbl1xr1 KO cells may explain the tendency of these cells to differentiate, mimicking a condition of mitogen withdrawal in NSCs in vitro (Chojnacki and Weiss, 2008), explaining the microcephaly in both our model and human patients mutated in TBL1XR1. Notably, MAPK activity has also been associated with neurite outgrowth and, accordingly, Tbl1xr1 KO neurons displayed a poorly developed dendritic tree.