Syndecan-1 is a major transmembrane proteoglycan and performs diverse functions in multicellular organisms, affecting the cell interactions, acting as a coreceptor to bind with many ligands and to integrate them with their receptors, playing a protective role, and influencing regeneration and tumorigenesis. Syndecan-1, as well as the total extracellular matrix, plays an important developmental role. The functional diversity is related with the syndecan-1 structure. The extracellular and cytoplasmic domains of syndecan-1 were found to be intrinsically disordered. This feature allows syndecan-1 to to bind with adapter proteins in the cytoplasm and be extended with glycosaminoglycans in the extracellular matrix, and to take part in the diverse and important cellular processes. The occurrences of 20 amino acids in syndecan-1 proteins from 32 animals were compared with those in 17 animal proteomes. Gly, Thr, Gln, Glu, and Pro were found to predominate in the former, facilitating the lack of an ordered structure.