The cDNA sequence and predicted protein sequence of the human and mouse genes are highly homologous, with 98% similarity over the entire predicted amino acid sequence. During mouse limb development at embryonic day 11.5, Six1 and Six2 are expressed distally over the posterior and anterior limb regions, respectively. However, by E14.5, expression of Six1 and Six2 is detected flanking the phalanges in domains corresponding to the anterior-posterior and dorsoventral axes, respectively. Six1 is also weakly expressed in skeletal muscles of the head and body during late development.human SIX1 homeobox gene from late S phase mammary carcinoma cells and demonstrated that overexpression of SIX1 leads to an abrogation of the DNA damage-induced G2 cell cycle checkpoint.