acetyl group is removed by one of the HDAC enzymes during deacetylation, allowing histones to interact with DNA more tightly to form compacted nucleosome assembly. This increase in the rigid structure prevents the incorporation of transcriptional machinery, effectively silencing gene transcription. Another implication of histone acetylation is to provide a platform for protein binding. As a posttranslational modification, the acetylation of histones can attract proteins to elongated chromatin that has been marked by acetyl groups. It has been hypothesized that the histone tails offer recognition sites that attract proteins responsible for transcriptional activation. Unlike histone core proteins, histone tails are not part of the nucleosome core and are exposed to protein interaction. A model proposed that the acetylation of H3 histones activates gene transcription by attracting other transcription related complexes. Therefore, the acetyl mark provides a site for protein recognition where transcription factors interact with the acetylated histone tails via their bromodomain. Histone code hypothesis The Histone code hypothesis suggests the idea that patterns of post-translational modifications on histones, collectively, can direct specific cellular functions. Chemical modifications of histone proteins often occur on particular amino acids. This specific addition of single or multiple modifications on histone cores can be interpreted by transcription factors and complexes which leads to functional implications. This process is facilitated by enzymes such as HATs and HDACs that add or remove modifications on histones, and transcription factors that process and "read" the modification codes. The outcome can be activation of transcription or repression of a gene. For example, the combination of acetylation and phosphorylation have synergistic effects on the chromosomes overall structural condensation level and, hence, induces transcription activation of immediate early gene. Experiments investigating acetylation patterns of H4 histones suggested that these modification patterns are collectively maintained in mitosis and meiosis in order to modify long-term gene expression. The acetylation pattern is regulated by HAT and HADC enzymes and, in turn, sets the local chromatin structure. In this way, acetylation patterns are transmitted and interconnected with protein binding ability and functions in subsequent cell generation. Bromodomain The bromodomain is a motif that is responsible for acetylated lysine recognition on histones by nucleosome remodelling proteins. Posttranslational modifications of N- and C-terminal histone tails attracts various transcription initiation factors that contain bromodomains, including human transcriptional coactivator PCAF, TAF1, GCN5 and CREB-binding protein (CBP), to the promoter and have a significance in regulating gene expression. Structural analysis of