The histone code is the pattern or sequence of histone modifications on a chromatin.
Histones play an important role in transcriptional activity and chromatin structure, altering the condensation state of the DNA. This can range from heterochromatin (generally perceived to be transcriptionally inactive) to euchromatin (understood to be transcriptionally active), with facultative and constitutive heterochromatin demonstrating the range of activity within the two classifications of chromatin. Facultative heterochromatin is a more transcriptionally active form of heterochromatin, allowing gene loops to enabling transcription to occur. Constitutive heterochromatin is more tightly packaged, and unable to be transcribed - this is used for X-inactivation, where the Barr body is unable to be transcribed.
The modification of histones can allow the recruitment of factors necessary for the up and downregulation of transcription.