MSC marker expression. Proliferating MSCs were analyzed for the surface expression of (A) CD105 (endoglin), CD73, CD106 (VCAM-1), CD29, CD44, and CD90, which are all constitutively expressed, and (B) HB-EGF, HER-1, and HER-4 molecules. In panels A and B, the x axis measures positive marker, open curves, cells positive for the marker; and shaded curves, negative controls. Only HER-1 is expressed in standard conditions. The presence of the mRNA for HB-EGF, HER-1, and HER-4 was evaluated by RT-PCR on MSCs (C). Lane 1 indicates negative control (water); lane 2, MSCs; lane 3, MSCs after the addition of 25 ng/mL HB-EGF to culture; lane 4, A431 cell line (HER-1 positive control); and lanes 5-6, U937 cell line (HB-EGF and HER-4 positive controls). Beta-actin is the quantitative control.