Ehlers-Danlos syndrome is a heterogeneous disorder characterized by joint hypermobility, skin hyperextensibility, fragility, and other signs of connective tissue involvement. In addition to these, the type VI variant of the disease has some special characteristics such as kyphoscoliosis and ocular abnormalities. The biochemical abnormality in most patients with this autosomal recessively inherited type VI variant is a deficiency in the activity of lysyl hydroxylase (EC 188.8.131.52), the enzyme catalyzing the formation of hydroxylysine in collagens and other proteins with collagen-like amino acid sequences. The type VI variant of Ehlers-Danlos syndrome was first identified in two sisters with a reduced amount of lysyl hydroxylase activity in their skin fibroblasts (S.R. Pinnell, S. M. Krane, J. E. Kenzora, and M. J. Glimcher (1972) N. Engl. J. Med. 286: 1013-1020). Our recent molecular cloning of lysyl hydroxylase has now made it possible to study the mutations leading to the deficiency in lysyl hydroxylase activity in these cells. Our data indicate that the mRNA for lysyl hydroxylase produced in the affected cells is about 4 kb in size, whereas it is 3.2 kb in the control cells. The sequencing of the cDNA for lysyl hydroxylase from the affected cells revealed an apparently homozygous duplication rearrangement of nucleotides 1176 to 1955, corresponding to amino acids 326 to 585 in the normal sequence. From Southern blotting data, the duplicated area in the gene equals about 6-9 kb and corresponds to seven exons.