The transient axonal glycoprotein (TAG-1) is a cell adhesion molecule that 
promotes neurite outgrowth and belongs to the immunoglobulin superfamily. We 
have isolated cDNAs encoding TAX1, the human homologue of TAG-1. Human TAX1 
shows a high degree of homology to rat TAX1 and less to its chick counterpart, 
axonin-1, with 91 and 75% identity at the amino acid level, respectively. The 
numbers of immunoglobulin (IgC2) domains and fibronectin repeats present in 
TAG-1 are conserved among the three species. The highest degree of conservation 
occurs in the second IgC2 domain (98% with the rat and 82% with the chick). The 
human homologue also contains a putative N-terminal signal sequence and a 
C-terminal hydrophobic sequence, suggestive of linkage to the cell membrane via 
phosphatidylinositol. In addition, the two mammalian TAG-1 proteins share the 
RGD tripeptide, a motif known to mediate recognition of fibronectin by 
integrins. In situ hybridization to human metaphase chromosomes maps the TAX1 
gene encoding human TAG-1 to a single location on chromosome 1q32.