Homozygous: For each gene, there are two versions, one inherited from the mother and one from father. In normal inheritance, the gene can either be dominant or recessive. A homozygote has identical copies of both genes, either recessive or dominant. In the context of this paper, a homozygous mutant female has lost both copies of the FosB gene. Both copies have been "knocked out".

Heterozygous: This is when the versions of a gene differ; sometimes one dominant, one recessive. For this paper, it means the mouse has one FosB gene, and the complimentary version is missing.

Genotype: The genetic makeup of the mouse

Wild-type: These are the "normal" mice.  A generic mouse.

Allele: One version of a gene.