Abstract Investigation and study of rare genetic disorders can lead to considerable and rapid advancements into understanding common and basic physiological processes. One such case is the study of the rare human disorder of sitosterolemia. This autosomal recessive disorder is characterized by the accumulation of plant sterols in the blood and tissues. The identification of the molecular defect(s) responsible for causing sitosterolemia has not only advanced our understanding of how we normally regulate dietary sterol entry, but may have also led to the identification of the long sought after transporters responsible for secretion of cholesterol (and other sterols) into bile. This review will summarize the history and current knowledge of how the genetic defects identified have improved our understanding of these important processes.