CerS4 activity is required to maintain membrane lipid homeostasis. (A) Schematic of the substrate specificity of CerS. (B) Ceramide levels in control (Ctr) and CerS4^epi−/− organoids determined by quantitative LC-ESI-MS/MS analysis. Note increased C16:0 and slightly decreased C18:0 and C20:0 ceramides (n = 6 mice/genotype; ratio-paired t test). (C) Levels of C16:0 and C20:0 ceramides in FACS-purified CD34⁺ integrin α6⁺ HFSCs and CD34⁻ integrin α6⁺ non-HFSCs determined by quantitative LC-ESI-MS/MS analysis (n = 5 mice/genotype; ratio-paired t test). (D) Sphingomyelin levels in Ctr and CerS4^epi−/− organoids determined by quantitative LC-ESI-MS/MS analysis. Note decreased C18:0, C18:1, C20:0, C22:0, C22:1, C24:1, C26:0, and C26:1 sphingomyelins (n = 6 mice/genotype; ratio-paired t test). LC-ESI-MS/MS, LC coupled to electrospray ionization tandem MS/MS.