This proposal was a preliminary assessment of the genetic impact of the Mx locus of salmonid fish on disease resistance/susceptibility of both cultured and wild fish. Specifically, it focused on the characterization of Mx genes and their potential use as genetic markers to assess the genetic impact of diseases on fish.  We have isolated an Mx gene from rainbow trout which is induced in vivo by IHNV infection.  The Mx gene is highly conserved in animals and its expression can be correlated to disease resistance.  Our major objective was to determine whether the expression of this gene correlated with resistance to IHNV and whether the trout Mx gene may be used as a genetic marker for disease resistance.  Rainbow trout from Clear Springs Trout Co., with proven susceptibility or resistance to IHNV were analyzed for their ability to express the Mx gene and whether this expression could be correlated to disease resistance. We found that almost all fish were expressing Mx mRNA at 48 hours post injection. We did not find a significant number of fish that could not express one or both of the Mx transcripts or expressed a truncated Mx transcript as found in experiments with inbred mice. The restriction fragment length polymorphism (RFLP) for the Clear Springs' rainbow trout were also analyzed. The HpaI enzyme digestion yielded different RFLP patterns even between individuals from the same population of rainbow trout. To determine whether Mx gene probes may be used as disease resistance genetic markers as well as for RFLP analysis, different stocks of rainbow trout, chinook, coho and kokanee were collected and examined. Again, all fish were able to express two Mx transcripts upon Poly I:C induction. Unlike the mouse system, all salmonids investigated are able to express full length Mx mRNAs. We also found that  there were different RFLP patterns among stocks of fish, but that some species of fish shared the same patterns.