Center of Regenerative Medicine in Barcelona, Doctor Aiguader 88, 08003 Barcelona, Spain.
A previous knockout of the transcription factor gene nuclear factor IX (NFIX) in mice produced impaired development of the corpus callosum and severe skeletal defects. A recent paper in BMC Developmental Biology reports an apparently similar NFIX knockout that produced marked differences in phenotype, raising intriguing general questions about the possible causes of such differences in mouse knockouts.