In non-eutherians, a third type of T cell The two established T cell lineages found in jawed vertebrates use either an αβ or a γδ T cell receptor (TCR) to detect antigens. Recently, another type of TCR chain (TCRµ) was found in marsupials and monotremes. Morrissey et al. analyzed T cells from the gray short-tailed opossum and uncovered a third lineage resident in the spleen that uses a γµ TCR (see the Perspective by Criscitiello). The authors then characterized the crystal structures of two different γµ TCRs, which exhibited an architecture distinct from αβ or γδ TCRs in which a highly diverse, unpaired immunoglobulin-like variable domain was predicted to be the major antigen recognition determinant. Like camelid VHH and shark IgNAR antibodies, γµ TCRs could potentially inform future nanobody development. Science , this issue p. 1383 ; see also p. 1308