RXLR is a family of phytopathogen avirulence or effector proteins. RXLR proteins are defined by a secretion signal peptide - not in this family - followed by a conserved N-terminal domain with the sequence motif RXLR (Arg-Xaa-Leu-Arg) consensus sequence. The RXLR part is required for translocation inside plant cells, although it appears to be dispensable for the biochemical activity of the effectors when expressed directly inside host cells. The effector activity resides in the C-terminal part of the family, which activate effector-triggered immunity in plants that carry a corresponding resistance (R) protein. The C-terminal region exhibits a fold appears to be able to evolve to outwit the host as the latter tries to acquire new immunity [1].

1: Structures of Phytophthora RXLR effector proteins: a conserved but adaptable fold underpins functional diversity. Boutemy LS, King SR, Win J, Hughes RK, Clarke TA, Blumenschein TM, Kamoun S, Banfield MJ; J Biol Chem. 2011;286:35834-35842. PMID:21813644