RNA was purified from cell lines using the QIAgen miRNeasy kit (Valencia, CA) and RT-PCR was performed to amplify FGFR3-L and FGFR3-S using random hexamers (Thermo Fisher Scientific). 5’ and 3’ rapid amplification of cDNA ends (RACE) was performed as previously described8 (link). RACE primers were designed based on the reference sequence of FGFR3 provided by the National Center for Biotechnology Information (NCBI) and are listed in Supplemental Table S1. RT-PCR products were ligated into pCR2.1-TOPO TA vector (Thermo Fisher Scientific) according to manufacturer’s instructions. Three to 4 clones per cell line were sequence verified (Sequetech, Mountain View, CA) and the consensus sequence of the full-length FGFR3-S variant was deposited in GenBank (Accession #MK542707).