Extracted ion chromatograms are shown for TUBL-derived peptides in MRM analysis. MRM analysis detected two different TUBL-derived peptides with the amino acid sequences AQLVGQGVSSWR (A) and DTLSDLR (B). “Standard” indicates signals for the internal standard, a stable isotope–labeled recombinant TUBL protein expressed in and purified from bacteria. “Endogenous” indicates signals for peptides derived from endogenous TUBL in epidermal lysates prepared from Tubl+/+ or Tubl−/− mice. The purple vertical lines in the Endogenous chromatograms indicate the retention time of each endogenous TUBL peptide.