Whilst it has been assumed that this is due to increased TSPO expression by activated pro-inflammatory microglia, recent studies of human MS brain tissue have shown that TSPO is expressed in a range of microglia phenotypes and that the increased TSPO signal in lesions predominantly reflects microglia/macrophage density rather than activation status or phenotype (Nutma et al., 2019, 2021). This evidence concerns the gene TSPO and myeloid sarcoma.