IFT172 and Bardet-Biedl syndrome: Cases in which cis-acting regulators of expression or trans-acting epistatic modifier alleles may be the origin of phenotypic variation are reported for BBS1 and IFT172. The BBS1 p.M390R variant was found in homozygosis in RP and BBS patients as well as in unaffected parents of BBS patients and in a control proband.