tokens: A genomic clone of the chicken osteopontin - encoding gene ( opn ) was isolated and found to be organized as follows : an untranslated 5 ' exon ; a signal peptide ; a recognition sequence for phosphorylation by casein kinase II ; a domain containing a possible O - linkage site for glycosylation ; a second casein kinase II phosphorylation site ; an exon containing three functional regions , the poly - Asp sequence of seven consecutive Asp residues , the RGD integrin recognition site and a potential N - linkage site for glycosylation ; and a large C - terminal exon which also contains a potential N - linkage site for glycosylation .