We have identified highly similar heterozygous COL6A1 genomic deletions, spanning from intron 8 to exon 13 or intron 13, in two patients with Ullrich congenital muscular dystrophy and the milder Bethlem myopathy. The 5′ breakpoints of both deletions are located within a minisatellite in intron 8. The mutations cause in-frame deletions of 66 and 84 amino acids in the amino terminus of the triple-helical domain, leading to intracellular accumulation of mutant polypeptides and reduced extracellular collagen VI microfibrils. Our studies identify a deletion-prone region in COL6A1 and suggest that similar mutations can lead to congenital muscle disorders of different clinical severity. We have identified similar heterozygous COL6A1 genomic deletions, spanning from intron 8 to exon 13 or intron 13, in two patients with Ullrich congenital muscular dystrophy and the milder Bethlem myopathy. The 5 ’breakpoints of both deletions are within a minisatellite intron 8 . The mutations cause in-frame deletions of 66 and 84 amino acids in the amino terminus of the triple-helical domain, leading to intracellular accumulation of mutant polypeptides and reduced extracellular collagen VI microfibrils. Our studies identify a deletion-prone region in COL6A1 and suggest that similar mutations can lead to congenital muscle disorders of different clinical severity.