TTLL9 and male infertility: Failure to maintain intact doublet structure was shown to be associated with infertility, e. g. the deletion of Ttll9 in mice caused doublet 7 shortening in the distal portion of the principal piece in sperm flagellum, along with reduction of doublet 5 polyglutamylation, leading to biased anti-hook bending and male infertility, similar to Cfap97d1 knockout males [45].