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.