Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant disorder 
linked to contractions of the D4Z4 repeat array in the subtelomeric region of 
chromosome 4q. By comparing genome-wide gene expression data from muscle 
biopsies of patients with FSHD to those of 11 other neuromuscular disorders, 
paired-like homeodomain transcription factor 1 (PITX1) was found specifically 
up-regulated in patients with FSHD. In addition, we showed that the double 
homeobox 4 gene (DUX4) that maps within the D4Z4 repeat unit was up-regulated in 
patient myoblasts at both mRNA and protein level. We further showed that the 
DUX4 protein could activate transient expression of a luciferase reporter gene 
fused to the Pitx1 promoter as well as the endogenous Pitx1 gene in transfected 
C2C12 cells. In EMSAs, DUX4 specifically interacted with a 30-bp sequence 
5'-CGGATGCTGTCTTCTAATTAGTTTGGACCC-3' in the Pitx1 promoter. Mutations of the 
TAAT core affected Pitx1-LUC activation in C2C12 cells and DUX4 binding in 
vitro. Our results suggest that up-regulation of both DUX4 and PITX1 in FSHD 
muscles may play critical roles in the molecular mechanisms of the disease.