FBN2 and Osteopenia: Loss of fibrillin-2 synthesis in Fbn2-/- mice is associated with several CCA traits, such as joint contractures, osteopenia and muscle weakness and atrophy, as well as a unique limb patterning defect not seen in either CCA patients or Fbn1-/- mice, namely digit fusion (syndactyly) with involvement of either soft or hard tissue [42].