sall4 acts downstream of tbx5 and is required for pectoral fin outgrowth.

Okihiro syndrome (OS) is defined by forelimb defects associated with the eye disorder Duane anomaly and results from mutations in the gene SALL4. Forelimb defects in individuals with OS range from subtle thumb abnormalities to truncated limbs. Mutations in the T-box transcription factor TBX5 cause Holt-Oram syndrome (HOS), which results in forelimb and heart defects. Although mutations in TBX5 result in HOS, it has been predicted that these mutations account for only approximately 30% of all individuals with HOS. Individuals with OS and HOS limb defects are very similar, in fact, individuals with mutations in SALL4 have in some cases previously been diagnosed with HOS. Using zebrafish as a model, we have investigated the function of sall4 and the relationship between sall4 and tbx5, during forelimb development. We demonstrate that sall4 and a related gene sall1 act downstream of tbx5 and are required for pectoral fin development. Our studies of Sall gene family redundancy and tbx5 offer explanations for the similarity of individuals with OS and HOS limb defects.