Abstract Lysyl hydroxylase (EC 1.14.11.4, procollagen-lysine 2-oxoglutarate 5-dioxygenase, PLOD) catalyses the formation of hydroxylysine in collagens and in the other collagen like proteins. Hydroxylysine participates in the formation of cross-links between collagen molecules and can bind to the carbohydrates, galactose and glucosylgalactose. Patients with the type VIA Ehlers-Danlos syndrome (EDS) have characteristically a deficiency in hydroxylysine of collagen in their skin that is caused by reduced activity of lysyl hydroxylase 1. In this work the mutations were studied in detail in four different Ehlers-Danlos VIA patients. The first patient characterized in this study had a duplication of seven exons in the lysyl hydroxylase gene 1. The mutation was caused by homologous recombination of two identical 44-nucleotide regions of Alu sequences in introns 9 and 16 in the gene. This study also suggests that uniparental isodisomy does not explain the homozygosity of the mutation. The second patient was found to have two mutations in the gene for lysyl hydroxylase 1 in a compound heterozygote state. The study resulted in the discovery of the first deletion mutation in the gene. The deletion was caused by an Alu-Alu recombination that removes about 3 kb from the gene including all the exon 17 sequences. The other mutation causes deletion of exon 16 from the mRNA. Deletion of the penultimate nucleotide of intron 15 destroys the consensus sequence of the intron/exon boundary and thus causes the deletion. The third patient was described to have a nonsense codon in exon 14 of one allele which causes a reduction in the amount of lysyl hydroxylase mRNA and leads to aberrant RNA splicing in the cell. The other allele was concluded to be operationally null. In the last work two novel null mutations were found in the gene for lysyl hydroxylase 1. The first was a one nucleotide deletion in the acceptor splice site of intron 4 and the other an insertion of a C nucleotide in exon 2. The abnormal alleles lead to markedly decreased lysyl hydroxylase mRNA levels. This work revealed many exon deleted splicing variants of lysyl hydroxylase mRNA which were first discovered in affected cells, but traces of similarly spliced mRNA species were also found in the cytoplasm of normal human skin fibroblasts. These data indicate that the splicing machinery of the cell is leaky. In this thesis, several types of stuctural mutations in the DNA were found to be responsible for lysyl hydroxylase deficiency in patients with type VIA variant of EDS. The different mechanisms causing these mutations were also studied in detail.