COL1A1 and cyst: Although Pkd1 is expressed in undifferentiated mesenchyme during embryogenesis [1], [2], [51] and Pkd1 mutations lead to cyst formation in the kidney, pancreas and liver in hereditary polycystic diseases [52], [53], [54], [55], [56], extraskeletal abnormalities due to deletion of Pkd1 in mesenchymal precursors using Col1a1(3.6)-Cre was unexpected.