Considering that many of the cataract-associated mutants characterized in exogenous expression systems show loss of function (and only some of them have a dominant negative effect on co-expressed wild type connexins) and that mice with half of the Cx46 or Cx50 levels (heterozygous null mice) do not develop cataracts [19, 94], it is intriguing that most Cx46 and Cx50 mutants cause dominant cataracts. This evidence concerns the gene GJA3 and cataract.