There are eight classes of semaphorins and four types of plexins. Semaphorin (SEMA) classes 1 and 2 are found in invertebrates and classes 3-7 are vertebrate sempahorines. Sempahorin class 3 is secreted, whereas the other classes are synthesised as transmembrane proteins. Vertebrate plexins (PLXNs) are classified into four subfamilies plexin-A to -D. There are four A-type plexins, three B-type, one C-type and D-type. Interactions between different subfamilies of plexins and semaphorins show differential specificity, which trigger different sets of biological functions. Another level of functional specificity specificity is attained by plexins by coupling with various coreceptors expressed in a cell- or tissue-specific manner, such as neuropilins (NRP), L1CAM, c-MET proto-oncogene, ERB2, CD72 and CD45 (Kruger et al. 2005, Law & Lee 2012).