Gene structure of selected members of the Fgf family. Only the portion of each gene containing coding exons is shown. Constitutively expressed exons are in black; alternatively spliced exons are in gray. Fgfs1, 2, 4 and 9 contain the prototypic three-exon organization. For Fgf1, 5' untranslated exons are not shown; inclusion of these exons extends the gene by approximately 69 kb . Fgf8 is an example of a gene with 5' alternative splicing, and Fgf13 demonstrates alternatively used 5' exons separated by over 30 kb. References: Fgf1 ; Fgf2 ; Fgf4 ; Fgf8 ; Fgf9 ; Fgf13 .