Segmental UPD of EFL1 in SDS: a schematic representation. Normal individual has 2 copies of wild-type (WT) EFL1 alleles (upper left). In the pathological situation (upper right), both EFL1 alleles carry deleterious/pathogenic variants (the X black allele is mildly defective, whereas the X red one is profoundly defective). Uniparental disomy (lower scheme) leads to the substitution of the most deleterious EFL1 allele (red X allele) by the less deleterious one (black X allele), contributing to mitigate the functional consequences of the most severe allele.