The Mla locus in barley (Hordeum vulgare ssp. vulgare) contains a large number of alleles with distinct recognition specificities against isolates of the fungal pathogen powdery mildew (Blumeria graminis f. sp. hordei). Classical genetic studies have found more than 30 Mla specificities in different barley cultivars. Previously, only six Mla alleles have been isolated and characterised at the molecular level. We isolated and characterised 23 new Mla sequences designated as candidate MLA cDNAs using a PCRbased approach. Thus, Mla currently contains a total of 29 different variants. Functional activity was demonstrated for 13 of the 23 candidate MLA cDNAs. Four Mla sequences encoded N-terminal coiled-coil (CC) regions that differed significantly from the Nterminus conserved among the other 25 MLA proteins. The MLA proteins are highly polymorphic in the C-terminal LRR (leucine-rich repeat) domain. Investigation of this region revealed 30 positively selected sites that lie mostly in the variable x positions of the 15 LxxLxLxx motifs. As the LRR domain is possibly determining recognition specificity, the large diversity of MLA will help to precisely identify the sequence requirements for the detection of the pathogenic effector proteins.