Autosomal dominant mutations in the gene encoding the basic helix-loop-helix 
transcription factor Twist1 are associated with limb and craniofacial defects in 
humans with Saethre-Chotzen syndrome. The molecular mechanism underlying these 
phenotypes is poorly understood. We show that ectopic expression of the related 
basic helix-loop-helix factor Hand2 phenocopies Twist1 loss of function in the 
limb and that the two factors have a gene dosage-dependent antagonistic 
interaction. Dimerization partner choice by Twist1 and Hand2 can be modulated by 
protein kinase A- and protein phosphatase 2A-regulated phosphorylation of 
conserved helix I residues. Notably, multiple Twist1 mutations associated with 
Saethre-Chotzen syndrome alter protein kinase A-mediated phosphorylation of 
Twist1, suggesting that misregulation of Twist1 dimerization through either 
stoichiometric or post-translational mechanisms underlies phenotypes of 
individuals with Saethre-Chotzen syndrome.