RSPH1 and primary ciliary dyskinesia: Moreover, in the tracheal cilia of the PCD patients, RSPH1 or RSPH4 loss-of-function mutation specifically abolished RS1 and RS2 heads, but not RS3 head (Lin et al., 2014; Zhao et al., 2021) even though RS3 shares T-shaped structure just like RS1 and RS2 in most eukaryotic motile cilia and flagella (Imhof et al., 2019; Leung et al., 2021; Lin et al., 2012; Lin et al., 2014).