HB-EGF inhibits OP-1 stimulation of aggrecan but not COL2A1, ID1, or SMAD6 expression. Chondrocytes were treated overnight with control media, 10ng/ml HB-EGF, 100ng/ml OP-1, or the same amounts of HB-EGF + OP-1. RNA was isolated and used for quantitative PCR with primers for aggrecan (A), COL2A1 (B), ID1 (C), and SMAD6 (D). Results shown are the mean±sem of samples from four independent donors.