caBMPR1A did not require specific endogenous type 1 receptors for its BMP-Smad signaling. Calvarial preosteoblasts were prepared from control (caBmpr1a(−);P0-Cre(+)) and mutant (caBmpr1a(+);P0-Cre(+)) mice, and expression of endogenous type 1 receptors were reduced by siRNA for Bmpr1a or Acvr1. Cells were treated by Noggin to block ligand-dependent BMP signaling to visualize BMP-Smad signaling transduced only by caBMPR1A. Vinculin was used for loading control. Levels of gene silencing in the mutant cells were measured by Q-RT-PCR. Scr, scrambled, *, p<0.01, N, not significant.