Expression of FGFR2 IIIb splice variant is characteristic of epithelial cells. A number of cis-acting elements have been identified in the FGFR2 pre-mRNA that are required for correct expression of the IIIb isoform and repression of the mesenchymal IIIc form (Muh et al, 2002; Hovhannisyan and Carstens, 2005; Hovhannisyan et al, 2006). These include the ISAR and ISE/ISS elements 1-3 in the region between exon 8 and exon 9 of the pre-mRNA. ESRP1 and ESRP2 are RNA-binding mRNA splicing factors that promote epithelial-specific IIIb splicing by binding to the ISE/ISS-3 sequence (Warzecha et al, 2009). A complex of RBFOX2, hnRNPH1 and hnRNPF may cooperate with the ESRP proteins to stimulate IIIb-specific splicing by binding to adjacent exonic GGG motifs (Baraniak et al, 2006; Mauger et al, 2008). This RBFOX2-hnRNP complex appears to compete with the IIIc-promoting trans-acting factor ASF/SF2 for binding to these exonic sites (Mauger et al, 2008). Other factors that appear to contribute to IIIb-specific splicing include hnRNPM, TIA1 and TIAL1, although their precise roles remain to be elucidated (Hovhannisyan and Carstens, 2007; Del Gatto-Konczak et al, 2000; Newman et al, 2006).