This mutation was induced by retroviral mutagenesis using an 8.0 kb myeloproliferative sarcoma retrovirus MPSV-mos-neo in mouse embryonic stem cells; an additional 154 bp of unique sequence also integrated at the locus during mutagenesis. The insertion site was identified as the intergenic region between Fgf3 and Fgf4. The retrovirus landed in the opposite transcriptional orientation to both genes and is approximately 10 kb downstream of Fgf3 and 5 kb upstream of Fgf4. Ectopic expression of both genes was up-regulated in the cranial suture region in mutant mice.