Transmembrane protein 119 (TMEM119) is a cell-surface protein which, in the brain, is found exclusively in microglia (Butovsky et al., 2014; Bennett et al., 2016; Satoh et al., 2016). Its function in microglia is not yet known, but in the periphery, this protein—also known as osteoblast induction factor—has roles in bone mineralization and testicular development (Mizuhashi et al., 2015). TMEM119 is often referred to as a marker of homeostatic microglia, as its expression is downregulated in various transgenic mouse models of neurodegenerative diseases, including Alzheimer’s disease (Keren-Shaul et al., 2017; Krasemann et al., 2017; Zhou et al., 2020). However, contrary to the observations in mice, levels of TMEM119 transcripts were found to be elevated in the brains of human Alzheimer’s subjects, compared with controls (Satoh et al., 2016; Zhou et al., 2020). Reports of protein levels in AD brains versus controls have been variable, with one study reporting an increase (Muñoz-Castro et al., 2022), another study reporting a decrease (Kenkhuis et al., 2022), and a third reporting no change (Satoh et al., 2016).