XLHers frequently experience calcified soft tissue, either in the kidneys (generally believed to be a side-effect of treatment) or in tendons and ligaments (the exact cause of which is unclear at present).
One such condition is OPLL or Ossification of the Posterior Longitudinal Ligament (the ligament that runs down the outside of the spine). These calcifications can occur in a variety of other places too, generally wherever tendons or ligaments attach to the bone. These calcifications are known as entheses and the condition is known as enthesopathy.
A recent journal article discusses whether the current treatment regimen (phosphorus and calcitriol supplements) reduces these calcifications, and unfortunately concluded that it does not. You can read it here:
And there's a related commentary here:
XLHers aren't the only ones who experience calcified tissues. There's a whole medical journal dedicated to them: Calcified Tissue, International (http://link.springer.com/journal/223).
You might think "calcified tissue" is a micro-specialty, but there's actually a European Calcified Tissue Society, which meets with the International Bone and Mineral Society every year in Europe (http://ects2017.org/), and there's also an Israel Society on Calcified Tissue Research (http://www.iofbonehealth.org/iof-national-societies/1140).