**6.1 Twins with keratoconus**

As early as 1954 occurrence of keratoconus in both identical twins had been reported [60]. Following twins reports in different ethnic groups, one twin pair was found to show different contrast sensitivities [61]. However, two pairs of Caucasian identical twins both showed similar features clinically and under videokeratography and radial keratotomy, as one pair were at early and the other as later stage [62]. Two pairs of monozygotic twins were found discordant for keratoconus in the USA, one from a hispanic family of Mexican descent and the other Caucasian from England [63], while dissimilarity in phenotype may suggest the absence of genetic involvement. However, natural monozygotic discordance could occur if there was separation of the zygote into two distinct cell masses after fertilization before the start of tissue differentiation. Post-zygotic events that lead to existence of two different cell clones in the early zygote may precede the twinning process. In the Mexican family, 39 members were examined and 5 were suspected for keratoconus by corneal topography. Also, one distant relative was a confirmed keratoconus patient. Corneal topography also revealed one suspect from 59 family members examined in the English family. There could be a genetic component in the keratoconus phenotype in these two families [63]. Of note, more and more concordant twins with keratoconus have been reported, including reported concordance in all 13 monozygotic and 5 dizygotic twins [64].
