This proposal aims to identify mechanisms underlying disease variability in soft tissue mineralization (STM), common in rare and acquired STM disorders. Among them pseudoxanthoma elasticum (PXE), caused by mutations in the liver transporter ABCC6, is a paradigm disorder characterized by multisystemic STM with significant and unexplained variability. Hence we use PXE as a model disease and will focus on its biochemical, clinical and molecular variability. First, we will investigate the mechanisms underlying the highly variable plasma levels of the calcification inhibitor inorganic pyrophosphate (biochemical variability) and the variable clinical severity in PXE using exome sequencing and transcriptomics in skin cells of the same patient. Next, we will study the mechanisms leading to incomplete penetrance (IP) of some ABCC6 mutations (molecular variability); we recently identified the first IP variant R391G as a result of our ongoing collaboration. We will identify other IP alleles and characterize them at the biochemical and cellular level. We will determine the genomic background necessary for these variants to become disease causing. In the third part, we will functionally validate the identified candidate modifiers for biochemical, clinical and molecular variability in different in vitro and in vivo assays.Together, the results of this proposal will lead to major advances in our knowledge on mechanisms underlying STM and will improve patient management and treatment.