Fibronectins (FNs) are adhesive and polymorphic glycoproteins present in biological fluids and extracellular matrices. These molecules are involved in several biological processes, such as cellular morphology, cellular migration, wound healing, angiogenic processes and neoplastic transformation. FN polymorphism is at least partially due to alternative splicing in three regions (IIICS, ED-A and FNIII-B) of the pre m-RNA coded for by a single gene. In all processes of tissue remodeling, both physiological (embryogenesis, wound healing, proliferative phase of endometrium) or pathological (for example, neoplasia), the pre-mRNA splicing pattern of FN is altered and generates an augmented expression of the isoforms containing the FNIII-B domain. In particular, the FN isoforms containing the FNIII-B domain (BFNs) are, with very rare exceptions, undetectable in normal adult tissues but show a considerable expression in fetal tissues, neoplastic tissues and tissues undergoing wound healing. Moreover, the accumulation of BFN around newly formed blood vessels makes it an important angiogenic marker.