BBS4 and Bardet-Biedl syndrome: In order to examine whether outer segment malformation is a phenotype specific to Bbs8-/- mice or a phenotype shared by mutant mice of BBSome components, we examined the outer segment ultrastructure of Bbs1M390R/M390R mice [20], which model the most common human mutation in BBS, and Bbs4-/- mice [19].