The invention concerns a ribozyme which is characterized in that it includes a catalytic zone (a) and a hybridization zone (b), the hybridization zone being essentially complementary either (1) to all mRNAs transcribed by the vertebrate genes of a specific family of closely related MHC alleles or (2) to mRNA transcribed by a vertebrate gene of an individual MHC allele and being able to selectively cleave this/these mRNA(s), the mRNA(s) including a target nucleic acid zone with the following properties: In case (1), the target nucleic acid zone is essentially conserved within all genes of a specific family of closely related MHC alleles, but differs in case (1) and (2) from genes of all other MHC alleles to an extent such that cleavage of the mRNAs transcribed by the genes of the other MHC alleles is eliminated by the ribozyme, thus making it possible to selectively reduce or inhibit the expression of all genes of a desired family of closely related MHC alleles or of the gene of an individual MHC allele by selective cleavage of mRNA(s) transcribed by these genes or this gene. Thus, at the transcription level, individually characteristic cell-surface molecules which are responsible for immunological rejection reactions can be deactivated, for instance for the purpose of rendering MHC-incompatible organs transplantable or treating an autoimmune condition. The invention also concerns drugs containing such ribozymes.
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