Biochemical properties of chromatin can be modulated by incorporation of histone variants in chromatin. MacroH2A has a amino-terminal domain typical of a full-length H2A, fused to a "macro domain". To date, three macroH2A have been identified: macroH2A1.1 and macroH2A1.2 which are alternative spliced variants and macroH2A2. They have been principally associated to heterochromatin formation and transcriptional repression. State of art shows that macroH2A1 is associated to various cellular mechanisms such as chromosome X inactivation, senescence, cellular development, transcriptional regulation and DNA double strand breaks. Many of these processes are implicated in cancer formation, making macroH2A1 an important element to consider. Its expression has also been correlated to numerous cancer developments such as lung, colon, skin or breast. Due to the structural differences, links to ligands and controversy observed in the literature between macroH2A1.1 and macroH2A1.2, it seems essential to study separately these two variants without omit their interconnection significance. Recently, we have determined that macroH2A1.1 expression level correlated specifically with Triple Negative Breast Cancer (TNBC). At the molecular level, this combination results in a positive correlation between macroH2A1.1 expression level and molecular characteristics of Epithelio-Mesenchymal Transition (EMT). In this context, the aim of my thesis was to identify biochemical properties of macroH2A1.1 in many breast cancer cellular models using a specific antibody for macroH2A1.1 generated in the lab. All this work has allowed the identification of a mono-ubiquitinated form of macroH2A1.1 on its lysine 123. Technical tools implemented have also allowed to identify the modified form in association with RNA:DNA duplexes. The results presented in this thesis will allow to better understand the importance of post-translational modification of histone variant and to open a new operating field in the role definition for this variant.