Rett syndrome is a severe neurological disorder caused by mutations in the MECP2 gene. The mutated gene can affect neurons and astrocytes, two important cell types in the brain. Here we showed that the gene is also important for the timing when these two cell types originated in the brain. We also found that this misregulation is caused by alterations in the LIN28 pathway, important for cell fate decision. The work reveals a novel role of MECP2 during development that might contribute and explain some clinical symptoms observed in affected individuals.