a) Primary microRNA (pri-microRNA) is processed by Drosha in the nucleus to form pre-microRNA. a) Primary microRNA (pri-microRNA) is processed by Drosha in the nucleus to form pre-microRNA. The pre-microRNA enters the cytoplasm via exportin 5 where it is further processed by Dicer into a mature microRNA duplex. One strand of this duplex is incorporated into the RNA-induced silencing complex (RISC) where it binds to the target mRNA. The mRNA is destabilised and, consequently, the protein is downregulated. Reproduced and modified from [14] with permission from the publisher. b) The two main components of the epigenetic code are DNA methylation, in which methyl (Me) groups added to certain bases repress gene activity, and histone modification, where a combination of chemical modifications of the histone “tails” alter the activity of the DNA wrapped around them. Reproduced and modified from [15] with permission from the publisher. Andrew Peacock Eur Respir Rev 2013;22:20-25 ©2013 by European Respiratory Society