Since Spp1 expression was seen in a subset of parenchymal microglia during the demyelination phase of the cuprizone model of MS in mice (32) and SPP1 was expressed in a subset of brain microglia associated with MS (32), it is possible that SPP1+ MG found in the CSF are correlates of parenchymal microglia designated for handling myelin debris. Here, SPP1 is linked to myeloid sarcoma.