- Variation in the vitreous phenotype of Stickler syndrome can be caused by different amino acid substitutions in the X position of the type II collagen Gly-X-Y triple helix. Richards, A.J., Baguley, D.M., Yates, J.R., Lane, C., Nicol, M., Harper, P.S., Scott, J.D., Snead, M.P. Am. J. Hum. Genet. (2000)