[29], [30] Thus, if the repeat expansion is causative for FECD, we hypothesize that the effect is to alter the expression of the gene in some way rather than to simply inactivate the gene as in Pitt-Hopkins syndrome. Given the variety of transcripts produced by this gene in a tissue specific manner, it is reasonable to consider the possibility that the repeat expansion alters either the transcription start site or level of expression of specific TCF4 isoforms that could be required for the maintenance of the corneal endothelium. Here, TCF4 is linked to Pitt-Hopkins syndrome.