Sry type HMG box (Sox6) is a member of the Sox transcription factor family characterized by the conserved high mobility group (HMG) domain, consisting of 79 amino acids involved in DNA recognition and binding [1]. Sox transcription factors bind to the minor groove of DNA and cause a 70°–85° bend of the DNA that leads to local conformational changes [2,3], while most other transcription factors target the major groove of DNA [4]. Therefore, Sox proteins may perform part of their function as architectural proteins by organizing local chromatin structure and assembling other DNA-bound transcription factors into biologically active, sterically defined multiprotein complexes. Sox6 has been reported to be able to act as either an activator or a repressor, depending on its interactors and its target promoter context [5,6]. Intriguingly, Sox6 has also been shown to act as a general splicing factor that participates in pre-mRNA splicing [7]. Depletion of Sox6 in HeLa cell extracts blocked splicing of multiple substrates, and expression of the HMG domain of either Sox6, Sox9, or Sry in the extracts restored splicing, indicating functional overlap of these proteins [7]. Regardless of how Sox6 functions in regulating gene expression, previous studies have demonstrated that Sox6 is an important regulatory molecule that plays a role in the development of the central nervous system [8–11], cartilage [6,12,13], and muscle [14,15]. A Sox6-null mutant mouse (p100H) has previously been identified in our laboratory [14]. Mice homozygous for p100H show delayed growth, develop myopathy and arterioventricular heart block, and die within 2 wk after birth [14]. The p100H mutant allele is associated with a Chromosome 7 inversion that disrupts both the p gene and the Sox6 gene (and no other gene within 50,000 nucleotides of the chromosomal breakpoints) [14]. Because the p gene functions solely in pigmentation [16], the Sox6 transcription factor is implicated in all other phenotypes.