Supplemental Figure S1. MALDI-TOF/TOF MS analysis of mouse serum glycans. MS/MS spectra were annotated using GlycoWorkbench software which calculates the percentage of experimental fragments that match with the theoretical fragments generated in silico. For each m/z value, the shown structure is the one which presented the largest number of fragments that match with the theoretical fragments. Only the B-, C-, Y- and Z-type ions are shown. The B- and C-type ions resulting from terminal reductive fragmentation are not systematically illustrated by the software. Most of the observed ions were sodiated. However, the sodiated ion with m/z 428 (NeuGc) has given rise to the ion with m/z 406, which is a desodiated and protonated form of the ion with m/z 428, and to the ion with m/z 374, which is a decarboxylated form of the ion with m/z 406. The ions with m/z 374 and 406 were frequently observed on the MS/MS spectra. Supplemental Figure S1

1580 Parent ion with m/z

1784 * The methodology for the extraction of N-glycans can introduce free O-glycans from serum. Therefore, ions 428, 877 and 953 likely are fragments of the parent ion 1807 that matches with a O-glycan ( ).  Ions 472 and 1389 are fragments of the parent ion Parent ion with m/z *877  472 *428  1389 *953  1795 *1807

1795 * Ions 428 and 877 are fragments of the parent ion  Ions 1506 and 1565 are fragments of the parent ion Parent ion with m/z *877  1506  1565  1784 *1807 *428

1836 * Ions 877 and 953 are fragments of the parent ion 1807 wich is likely a O-Glycan (see parent ion 1784). Remark: more fragments match with a structure showing a fucose on the terminal GlcNAc rather than a structure with a fucose on the reductive terminal GlcNAc. Parent ion with m/z *877 *1807 *953

1988 * Ions 374, 406, 472, 877, 1606 and 1795 are fragments of the parent ion  Ions 1576 and 1593 are fragments of the parent ion Parent ion with m/z  1999 *2012 *1795 *1606  1593  1576 *877 *472 *406 *374

1999 * Ions 877 and 1606 are fragments of the parent ion 2012 Parent ion with m/z *877 *1606 *2012

2012 * Ion 1592 is a fragment of ion Parent ion with m/z *1999 *1592

2040 *428 *877 *1157 *2012 * Ions 428, 877 and 1157 are fragments of the parent ion 2012 Parent ion with m/z

2192 * 374, 406, 428, 472, 877, 1780 and 1998 are fragments of the parent ion 2186, a monoantennary fucosylated and sialylated (NeuGc) glycan. Parent ion with m/z *2186 *374 *406 *428 *472 *877 *1780 *1998

2216 * 1317, 1781 and 1837 are fragments of the parent ion Parent ion with m/z *2244 *1837 *1781 *

2244 * 374, 406, 650, 877, 1852, 1980 and 1999 are fragments of the parent ion  1361 can be attributed to the protonated Z-type ion that comes from ion 2257 (see parent ion with m/z 2257). Parent ion with m/z *406 *374 *877 *650 *1852 *1980 *1999 *2257  1361

2257 *2244  1361 * * Ion 1837 is a fragment of the parent ion  Ion 1361 can be attributed to the protonated Z-type ion: Parent ion with m/z

2396 *877 *472 *428 *406 *374  1268 *2420 *1565 *1592 *2144 *1984 *1995 *2014 * Ions 374, 406, 428, 472, 877, 1565,1592, 1984, 1995, 2014,2144 are fragments of the parent ion  Ion 1268 was not able to be attributed. Parent ion with m/z

2420 *2448 *2042 * Ion 2042 is a fragment of the parent ion BY CZ Parent ion with m/z

2448 *2461 *877 *650 * Ions 650 and 877 can be attributed to ion Remark: more fragments matched with a structure displaying a fucose on antennae with Gal-Gal motif. Parent ion with m/z

*2448 *2042 * Ion 2042 is a fragment of the parent ion Parent ion with m/z

2607 *2635 *2230 *2173 *374 *406 *472 *877  855 *1081 *1317 * Ions 374, 406, 472, 855, 877, 1081, 1317, 2173 and 2230 are fragments of the parent ion  Ion 855 is a protonated and desodiated form of the ion 877. Remark: More ions match with a structure showing a substituted LacNAc antennae instead of a polysialylated structure. Parent ion with m/z

2635 *406 *374  196 *855  1752  1998 * Ions 374, 406 and 855 are fragments of the parent ion  Ions 196, 1752 and 1998 were not able to be attributed to a parent ion. Parent ion with m/z

*2809 *877 *650 *1720 *1983 *2433 *406 *374  1143  1593  2448  2822 § 819 *855 *428 § * Ions 374, 406, 650, 855, 877, 1720, 1983 and 2433 are fragments of the parent ion  Ions 1143, 1593, 1998 and 2448 are fragments of the parent ion § Ions196 and 819 were not able to be attributed to a parent ion.  1998 Parent ion with m/z

2809  406  374 *939 *1143 *1592 *1793 *1998 *2244 *2448 *2822 * Ions 939, 1143, 1592, 1793, 2244 and 2448 are fragments of the parent ion  Ions 374 and 406 are fragments of the parent ion Parent ion with m/z

Parent ion with m/z

2837 * Ions 428, 877, 1081, 1143, 1592, 1793, 1998, 2242, 2447 and 2576 are fragments of the parent ion *428 *877 *1081 *1143 *1592 *1793 *1998 *2242 *2447 *2576 *2852 Parent ion with m/z

2852 Parent ion with m/z

2867 *877 *1143 *2852 *1997 *2447 * Ions 877, 1143, 1997 and 2447 are fragments of the parent ion Parent ion with m/z

2895 *877 *939 *2056 *2448 *2505 * * Ions 877, 939, 2056, 2448 and 2505 are fragments of the parent ion Parent ion with m/z

2910 Parent ion with m/z

Parent ion with m/z

3056 *3026 *2622 * * Ions 1967, 2173 and 2622 are fragments of the parent ion  Ion 855 is a protonated and desodiated form of ion 877.  855 *1967 Parent ion with m/z

3228 *3244 *2839 *1998 *1268 *877 *650 *406 *374 * Ions 374, 406, 650, 877, 1268, 1998 and 2839 are fragments of the parent ion Parent ion with m/z

3244 Parent ion with m/z

3259 *3244 *2839 *1997 *1793 *1268  406  374 * Ions 1268, 1793, 1997 and 2839 are fragments of the parent ion  Ions 374 and 406 are fragments of the parent ion Parent ion with m/z

3288 *3244 *2839 *1997 *1268  406  374 * Ions 1268, 1997 and 2839 are fragments of the parent ion  Ions 374 and 406 are fragments of the parent ion Parent ion with m/z

3302 *3244 *2839 *1997 *1268 * Ions 1268, 1997 and 2839 are fragments of the parent ion Parent ion with m/z

3635 Parent ion with m/z *3663 *2839 *877 *650 *428 *406 *374 * Ions 374, 406, 428, 650, 877 and 2839 are fragments of the parent ion 3663.

* 2839 * 3288  406  374 Two potential structures (1 and 2) were observed, but more fragments matched with the structure 1 than the structure 2. * Ions 2839 and 3288 are fragments of the parent ion n°2.  Ions 374 and 406 are related to NeuGc sialic acid Parent ion with m/z

3678 *3693 *3288 *2839 *1983 *1268  406 * Ions 1268, 1983, 2839 and 3288 are fragments of the parent ion  Ion 406 is related to NeuGc sialic acid. Parent ion with m/z

3693 Parent ion with m/z 406

*1268*1984 *2433 *3288 *3693 * Ions 1268, 1984, 2433 and 3288 are fragments of the parent ion Parent ion with m/z

*406 *374 *650 *877 *2837 *3230 * Parent ion with m/z * Ions 374, 406, 650, 877, 2837, 3230 are fragments of the parent ion 4128 ( )