- Variation of the CGG repeat at the fragile X site results in genetic instability: resolution of the Sherman paradox. Fu, Y.H., Kuhl, D.P., Pizzuti, A., Pieretti, M., Sutcliffe, J.S., Richards, S., Verkerk, A.J., Holden, J.J., Fenwick, R.G., Warren, S.T. Cell (1991)