IPAS mapping of splice isoform variants for the MHC H2 K1 K gene. Intact proteins were subjected to anion exchange (AX) and reverse phase (RP) chromatography fractionation. Each fraction was then subjected to tryptic digestion and LC-MS/MS analysis. Peptides derived from novel exons falling within the MHC 2 K1 K gene were identified. Spectral counts for matching peptides derived from this gene are plotted as a function of AX and RP fractionation. The color of each cell indicates the number of matching spectra: black = high; gray = medium; light gray = low; white = zero. (a) The distribution of spectra matching the major annotated splice isoform (ENSMUST00000025181). (b) The distribution of spectra matching the novel splice isoform (M17C4317_13_s2_e917_1_rf2_c1_n0|). (c) The distribution of spectra matching peptides found in both the major annotated splice isoform for this gene (ENSMUST00000025181) and the novel splice isoform (M17C4317_13_s2_e917_1_rf2_c1_n0|).