Cilia were once thought to be evolutionary rem nants, but structural defects reveal their impor tance in signaling pathways and human disease, such as Joubert syndrome. Either of the genes TMEM138 and TMEM216 can be found mutated in phenotypically indistinguishable ciliopathy pa tients. Interestingly, despite their lack of sequence homology, these genes have always been aligned in head-to-tail configuration during vertebrate evolution.
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