Since Yadav et al. [17] have shown that Lrp5 produced in the intestine can inhibit Tph1 expression, and consequently also 5-HT synthesis and bone formation, we examined whether the LRP5 mutations causing primary osteoporosis influence Tph1 and/or 5-Htr1b expression in an in vitro system. Here, TPH1 is linked to osteoporosis.