One explanation for the apparent lack of differentiation of the grafted GFP+ cells in the mouse host cortex could be that NKX2.1 is downregulated following maturation, as occurs in mouse cortical interneurons (Marin et al., 2000), resulting in the loss of GFP expression. To test this possibility sections were examined for expression of SC121, a human-specific pan-neuronal marker (Kelly et al., 2004). As expected, all GFP+ cells also expressed SC121 (Figure S3). At 6 weeks after transplantation, less than 10% of human cells were single positive for SC121 and lacked GFP expression (25 out of 372 SC121+ cells). In sum, perhaps due to the protracted rate of cortical interneuron maturation in vivo, transplantation studies did not result in mature cortical interneurons within the first two months of transplant.
