Bet v 1, the major birch pollen allergen, is a member of a multigene family; a number of isoforms and homologous proteins from closely related species (alder, hazel and hornbeam) has been isolated and their cDNAs cloned and characterized. Genomic clones coding for Bet v 1 and homologues from apple and hazel were isolated and sequenced. Some of these clones contained intervening sequences. The exon-intron formation is highly conserved throughout this family of pathogenesis-related proteins in dicot plants and is also found in Aopr1 (Asparagus officinalis), a monocol species. Phylogenetic analysis suggested a possible common origin of the intron position in these homologous proteins at codon 62 in various families of flowering plants, including Fagaceae, Rosaceae and Apiaceae. This conserved 'proto-splice site' may point to a structure/function relationship. A conserved sequence motif (P-loop) was also found in all members of this protein family. Moreover, there is a certain degree of sequence similarity among the proteins derived from various species throughout the dicots and the only monocot examined. This fact is reflected by cross-reactivity from monoclonal and polyclonal antibodies raised against Bet v 1.