The comparative analysis of the human and mouse genomes has resulted in the identification of a large number of evolutionarily conserved sequences. The occurence of these so-called Conserved Non-Coding sequences (CNCs) has been further confirmed with the release of additional genomes. The strong evolutionary conservation of these sequences suggest they could contribute to the pool of yet non-identified functional sequences in the genome. This thesis has specifically focused on the functional analysis of a subset of HSA21 CNCs. The questions that were addressed are: i) what is the regulatory potentiel of CNCs in human cell lines, ii) what is the impact of phylogenetic conservation on their regulatory potential and iii) could CNCs contribute to human diseases?. Our study contributes to the functional annotation of CNCs by showing that the main role of mammalian CNCs might not be a regulatory one, however increasing the phylogenetic conservation favours a regulatory function.