Monozygotic twins are rarely completely identical despite their origin from a single zygote. The twinning process itself, or the very early intrauterine environment must provide the clues to explain this developmental dilemma. We present here discordant monozygotic twin girls, one of whom was diagnosed having IVIC (Oculo-oto-radial) syndrome on the basis of hand abnormalities and hearing loss but her twin sister has only strabismus. The family has at least 7 apparently and 2 possible affected members (gene carriers) over four generations, the majority being only midly affected. The 2 girls, whose monozygosity has been proved using independent DNA loci, show marked variability in expression, showing that for this gene modification of expression must be epigenetic or environmental rather than genetic.