These disagreements linking Rho/ROCK and bone development could be due to the need for a balanced signal and crosstalk between Rho/ROCK and growth factors rather an all-or-nothing response. RhoA expression was also significantly upregulated in bone tissues of Arhgap28-null mice. Due to the large family of mammalian RhoGAPs, it was predicted that Arhgap28-deficiency might be compensated for by functional redundancy between Arhgap28 and other RhoGAPs. Indeed, absence of Arhgap6 in mice does not cause an overt phenotype, presumably because of the observed compensatory mechanisms. It was surprising in our studies that Arhgap18 was not also upregulated because both Arhgap6 and Arhgap18 have been shown to negatively regulate RhoA and actin Cinepazide maleate stress fibers. These results reveal the potential of a novel co-regulatory mechanism for RhoA signaling and actin stress fibers by Arhgap6 and Arhgap28. How these RhoGAPs are activated during the patterning of ECM is unknown. Matrix assembly and detection of the ECM occurs at cell-matrix adhesion sites, therefore it is hypothesized that Arhgap28 is activated by signals downstream of cell-matrix adhesions in similar mechanisms described for other RhoGAPs. For examples, upon activation of integrin b1, p190RhoGAP is activated via tyrosine phosphorylation at the Nterminus by Src, and more recently, the identification of protein-protein Catharanthine interaction domains within DLC1 suggest direction interaction between DLC1 and cell-matrix adhesion proteins tensin, talin and focal adhesion kinase. In conclusion, we describe experiments that point to Arhgap28 being a functional negative regulator of RhoA during the formation of actin stress fibers in cells of mesenchymal origin. However, knockout experiments in mice indicated that Arhgap28 is not autonomous but is functionally redundant in the presence of other RhoGAPs including Arhgap6 and possibly Arhgap18. Such redundancy presumably underpins the cell��s ability to generate actin stress fibers during tissue formation. Plants sense diverse light signals from the environment via a family of plant photoreceptors, including phytochromes, cryptochromes, and phototropins.