A virus of yeast (S. cerevisiae)that results in the killing of other, usually non-infected yeast cells. The virus is a cytoplasmically inherited double-stranded RNA virus.

There are actually two virus genomes involved in killer virus: the L-A genome, which is replication-competent and serves as a helper virus, and is present in most yeast cells; and the M genome, which is a satellite (ie. not replication-competent) and requires the L-A genome to replicate, and is present in all killer strains. The L-A genome is allelic; a given yeast strain will only carry one allele of the L-A virus, most commonly L-A-H or L-A-HN. The M virus is also allelic; a given strain can carry only either M1 or M2. No allele on either virus is dominant, and the alleles can mix equally successfully. (It happens that the L-A-H and M1 tend to combine more often than L-A-H and M2, but it appears to be purely coincidental.) Individual alleles are exclusive in a cell, meaning that if an M1 cell is infected with the M2 virus, the viruses will compete with each other and one will be destroyed or released from the cell, while the other stays resident. Usually M1 happens to outcompete M2.

The M genome carries a gene for a preprotoxin, which when processed produces a toxin and a toxin resistance protein, allowing the cell to kill any cell that does not carry the M virus while preserving its own kind. Thus, a "killer" strain would have the phenotype K+R+, and a typical "non-killer" strain would be K-R-. (Other phenotypes can occur, but rarely.) The type 1 and type 2 toxins are different, meaning that a cell carrying M1 is still sensitive to the type 2 toxin and vice versa.

The virus is cytoplasmically inherited, which means that the virus resides in the cytoplasm and can be transmitted to another cell without exchange of nuclear material. (This was shown through cytoduction, or plasmogamy of cells without karyogamy, followed by a sporulation-type event.)

The yeast can be "cured" of the virus by growth at 37 degrees Celsius for at least 24 hours.

The virus is useful in examining certain phenomena of yeast genetics, including cytoplasmic inheritance, segregation, and epigenetic phenomena.