Homo- and hetero-oligomeric interactions between the transmembrane (TM) helices of integrin α and β subunits may play an important role in integrin activation and clustering. As a first step to understanding these interactions, we used the TOXCAT assay to measure oligomerization of the wild-type α_IIb TM helix and single-site TM domain mutants. TOXCAT measures the oligomerization of a chimeric protein containing a TM helix in the Escherichia coli inner membrane via the transcriptional activation of the gene for chloramphenicol acetyltransferase. We found the amount of chloramphenicol acetyltransferase induced by the wild-type α_IIb TM helix was approximately half that induced by the strongly dimerizing TM helix of glycophorin A, confirming that the α_IIb TM domain oligomerizes in biological membranes. Mutating each of the α_IIb TM domain residues to either Ala, Leu, Ile, or Val revealed that a GXXXG motif mediates oligomerization. Further, we found that the residue preceding each glycine contributed to the oligomerization interface, as did the residue at position i + 4 after the second Gly of GXXXG. Thus, the sequence XXVGXXGGXXXLXX is critical for oligomerization of α_IIb TM helix. These data were used to generate an atomic model of the α_IIb homodimer, revealing a family of structures with right-handed crossing angles of 40° to 60°, consistent with a 4.0-residue periodicity, and with an interface rotated by 50° relative to glycophorin A. Thus, although the α_IIb TM helix makes use of the GXXXG framework, neighboring residues have evolved to engineer its dimerization interface, enabling it to subserve specific and specialized functions.