Twist1 is the mouse ortholog of TWIST1, the human gene mutated in 
Saethre-Chotzen syndrome. Previously, a Twist1 null allele was generated by gene 
targeting in mouse embryonic stem cells. Twist1 heterozygous mice develop 
polydactyly and a craniofacial phenotype similar to Saethre-Chotzen patients. 
Mice homozygous for the Twist1 null allele die around embryonic day 11.5 (E11.5) 
with cranial neural tube closure and vascular defects, hindering in vivo studies 
of Twist1 function at later stages of development. Here, we report the 
generation of a Twist1 conditional null allele in mice that functions like a 
wild-type allele but can be converted to a null allele upon Cre-mediated 
recombination.