Despite the fact that the increase in OPN expression was reported to correlate with bone formation during valve calcification, also including the co-presence of bone marrow or cartilage [51,52,53], it should be pointed out that in this study, as well as our previous investigations on AVIC calcification, osseous or cartilaginous ectopic foci were never encountered [24,25,26,27,28,29,30,31,32,33]. Here, SPP1 is linked to calcification.