Gene duplications restricted to single lineage combined with an asymmetric evolution of the resulting genes may play particularly important roles in this lineage's biology. We searched and identified asymmetrical evolution in nine gene families that duplicated exclusively in rodents and are present as single-copies in human, dog, cow, elephant, opossum, chicken, lizard, and Western clawed frog. Among those nine gene families are Fas apoptosis inhibitory molecule (Faim), implicated in apoptosis, and Sperm antigen 6 (Spag6), implicated in sperm mobility. Both genes were duplicated in or before the Muroidea ancestor. Due to the highly asymmetric evolution of the resulting paralogs, the existence of these duplications had been previously overlooked. Interestingly, Spag6, previously regarded and characterized as a single-copy ortholog of human Spag6, turns out to be a Muroidea-specific paralog. Conversely, the newly identified, highly divergent Spag6-BC061194 is in fact the parental gene. In consequence, this gene represents a rare exception from the general rule of rapid evolution of derived rather than parental genes following gene duplication. Unusual genes such as murine Spag6 may help to understand which mechanisms are responsible for this rule.