mutations result in a weakening of the disulfide bond between the m protein and gp5 these constructs show in general normal replication and expression of the structural proteins resulting in an immune response comparable to wild type however fewer virus particles will be produced also it results in the production of virus particles which are impaired in the infection of the macrophage in both cases it results in a virus which is considered to be a safe vaccine for protection of pigs against for example prrsv our results also showed that mutations in the ectodomain of the m protein can result in the generation of a marker vaccine since replacement with the ldv ectodomain as well as deletion of some of its amino acids such as the deletion of the cysteine residue resulted in the loss of the binding of two mabs so mutation of the virus at this epitope results in the generation of a marker vaccine in this study we also showed that prrsv transcripts containing the ectodomain of the m protein of ldv generated an infectious chimeric virus also useful as a marker vaccine first the cysteine residues at position 50 111 and 117 in gp5 were substituted by serine residues for subsitution of amino acid 50 pcr mutagenesis was performed with primers lv32 and lv303 for the first fragment.