SPI1 and Huntington disease: Previous studies suggested that increased microglial expression of the myeloid lineage-determining factor PU.1 (encoded by the Spi-1 gene)—a master regulator of microglia development and function—might underly an exacerbated response to pro-inflammatory stimulation in HD microglia from knock-in (Q175) and fragment (R6/2) HD models [12].