There is provided a novel use for vascular endothelial growth factor, herein designated VEGF-X, and a CUB domain present in the sequence of VEGF-X, which enhance smooth muscle cell proliferation and can be used to treat diseases associated with reduced smooth muscle cell proliferation. VEGF-X, and a CUB domain can also be used in tissue engineering applications to increase the number of smooth muscle cells within specific tissue to restore that tissue function or architecture. Screening methods for identifying inhibitors of VEGF-X biological activity are also disclosed and these inhibitors include neutralizing VEGF-X antibodies, antisense VEGF-X sequences or non-protein antagonists competing with VEGF-X biological activity. Also provided are therapeutic methods for treating disorders associated with smooth muscle cells hyperproliferation and methods of diagnosis a pathological condition or susceptibility to a pathological condition associated with smooth muscle cell hyperproliferation.