Recent studies have shown that membrane lipids such as glycosphingolipids form distinct and functional microdomains and/or nanodomains. Although extensive studies have characterized membrane lipids, the precise composition of these domains is still not fully elucidated. In 2003, we identified a novel glycolipid, phosphatidylglucoside (PtdGlc), and categorized it as a new member of the lipid microdomain (Nagatsuka et al. 2003). PtdGlc, which is enriched in a detergent-insoluble membrane fraction, appears to be involved in cellular differentiation. Its chemical structure, however, was not fully studied. Recently, the complete chemical structure of PtdGlc isolated from rat embryonic brain tissues has been determined. Surprisingly PtdGlc is exclusively composed of saturated fatty acyl chains (C18 : 0/C20 : 0) in its diacylglycerol moiety (Nagatsuka et al. 2006a). The saturated fatty acid composition with single molecular species is barely detectable in known mammalian lipids. Very recently, we found that PtdGlc is a novel cell surface marker for developing radial/astro glia cells, and lyso-derivative secreted from the glia cells has potent biological activities to surrounding neurons. Here, we present an improved method to detect PtdGlc in brain tissues by MS spectrometry (Nagatsuka et al. 2006b).