Fig. 1.

(A) Two parallel systems of adaptive immune receptors have evolved in the jawed and jawless vertebrates. Whether all three VLRs are present in hagfish awaits further genomic analysis (*). (B) The three types of VLR have unique characteristics and are expressed in distinct lymphocyte populations. (C) VLRC is assembled within the genome using a series of LRR-encoding cassettes. The structure of the proteins encoded by mature VLR transcripts and an abstraction of the VLRC genomic locus are shown. The number of each of the cassette types is indicated. CP, connecting peptide; LRR1, a short first LRR; LRRCT, C-terminal cap; LRRNT, LRR antigen binding region composed of an N-terminal cap; LRRV, variable LRRs.