Some intriguing changes in the gene structure of DSCAM have occurred in arthropods where several duplications of exons generated three large tandem arrays that are alternatively spliced. This alternative splicing of individual exon sequences within an array occurs in a mutually exclusive and combinatorial manner allowing for expression of tens of thousands of Dscam isoforms. In the arthropods' genomes these three large exon arrays encode the N-terminal halves of the second and third Ig domains and the full Ig7 domain. The different structures of these isoforms lead to differences in binding interactions. Crystal structures of two "D. melanogaster" isoforms (with the first four Ig domains only), D9.9 and D1.34 () shows large variations in their binding epitopes and dimerization interface and conformations. Much of the difference is found the Ig3 domain loop.