TMEM121 and Astigmatism: For example, CLSTN2 (calsyntenin 2) lies on chromosome 3 and is relevant to astigmatism; TMEM258 (transmembrane protein 258) is related to spinocerebellar ataxia; another gene, TMEM121 (transmembrane protein 121), plays an essential role in the membrane; and FEN1 (flap structure-specific endonuclease 1) is linked to Xeroderma pigmentosum through base excision repair [32,42].