Alternatively, absence of an unchallenged pulmonary phenotype could result from a milder defect in mucociliary clearance in the lower airway that is related to the functions of Pcdp1 and Spef2, which is supported by the presence of lung disease in unchallenged mice lacking other ciliary proteins, such as Spag17 or both Spag6 and Spag16L [35,38]. This evidence concerns the gene SPAG6 and lung disorder.