Journal of Cell Science 1724 including cell-adhesion molecule L1 (Castellani and Rougon, 2002) and the receptor tyrosine kinases (RTKs) VEGFR2 (Toyofuku et al., 2004), erythroblastic leukemia viral oncogene homolog 2 (ErbB2) (Swiercz et al., 2004), off-track kinase (OTK) (Winberg et al., 2001) and Met (also known as HGFR) (Giordano et al., 2002). In the immune system, CD72 and Tim2 (T-cell immunoglobulin and mucin domain-containing protein 2) were found to interact functionally (although with low affinity) with the transmembrane semaphorins SEMA4D and SEMA4A, respectively (Kumanogoh et al., 2002;Kumanogoh et al., 2000). Recently, other ligands for NPs have also been described, including fibroblast growth factor 2 (FGF2) (West et al., 2005), hepatocyte growth factor (HGF) (Sulpice et al., 2008) and galectin-1 (Gal-1) (Hsieh et al., 2008). In addition, NPs were recently observed to form complexes with additional cell-surface receptors, including Met (Matsushita et al., 2007), β1 integrin (Fukasawa et al., 2007) and transforming growth factor-β1 (TGF-β1) (Glinka and Prud'homme, 2008) (for reviews see Neufeld and Kessler, 2008;Pellet-Many et al., 2008).In addition to their function in a range of basic cellular processes, recent studies have shown that semaphorin-mediated signals might also play important regulatory functions in cancer . On one side, tumor progression and metastatic dissemination depend on intrinsic properties of cancer cells, such as survival, self-renewal and the ability to migrate and overcome tissue barriers (invasiveness). On the other side, the tumor stroma -which includes endothelial cells, fibroblasts and cells of the immune system -is engaged in active molecular crosstalk with cancer cells. For example, blood-and lymph-vessel angiogenesis, together with inflammatory and immunosuppressive responses, further promotes cancer-cell survival, migration and invasion, as well as initiation of the metastatic cascade. In this Commentary, we review the current knowledge on the emerging role of semaphorin signals in controlling these 'two sides of a coin' -that is, the tumor-cell intrinsic alterations, and the crosstalk between cancer cells and other cells of the tumor microenvironment. In addition, the implications of semaphorin signaling in tumor progression will be discussed. . Of the vertebrate semaphorins, those in classes 4, 5 and 6 are transmembrane proteins, whereas those in class 7 are membrane-anchored via glycophosphatidylinositol (GPI). Class-3 secreted semaphorins have C-terminal basic-charged sequences, which are required for binding to neuropilins. Several class-3 semaphorins and SEMA4D have been shown to undergo regulatory cleavage by furins or metalloproteases. Class-5 semaphorins are distinguished by thrombospondin repeats. Several semaphorins also contain immunoglobulin-like domains. (B) Neuropilins are transmembrane receptors that are characterized by two complement-like (CUB) domains (also called the a1 and a2 domains), two FV/FVIII coagulation factor-like domains (also called the...