SPI1 and frontotemporal dementia: At a mature stage of 40 days in vitro (DIV), both control and C9orf72 ALS/FTD iPSC-MG display a typical ramified microglia morphology (Figure 1B) and express classic microglial marker proteins including the myeloid transcription factor PU.1, purinergic surface receptor P2RY12, and C-X3-C Motif Chemokine Receptor 1 (CX3CR1), as confirmed via fluorescent immunostaining (Figure 1C, Supplementary Figure S1A).