Splice isoforms lacking the IgI domain and/or the IgI-IgII linker region of FGFRs also occur naturally (Fig. 3F), the most important of which is FGFR1β, which lacks the IgI domain and is upregulated in a range of cancers, including astrocytoma, pancreatic, breast, and bladder cancer [105–108]. Here, VPREB1 is linked to urinary bladder cancer.