The inner nuclear membrane protein MAN1 has been identified as an important factor in transforming growth factor beta/bone morphogenic protein (TGFbeta/BMP) signaling. Loss of MAN1 results in three autosomal dominant diseases in humans; all three characterized by increased bone density. Xenopus embryos lacking MAN1 develop severe morphological defects. Both in humans and in Xenopus embryos the defects originate from deregulation of TGFbeta/BMP signaling. Several independent studies have shown that MAN1 is antagonizing TGFbeta/BMP signaling through binding to regulatory Smads. Here, recent progress in understanding MAN1 functions is summarized and a model for MAN1-dependent regulation of TGFbeta/BMP signaling is proposed.