H3 and H4 histone proteins are the primary targets of HATs, but H2A and H2B are also acetylated "in vivo". Lysines 9, 14, 18, and 23 of H3 and lysines 5, 8, 12, and 16 of H4 are all targeted for acetylation. Lysines 5, 12, 15, and 20 are acetylated on histone H2B, while only lysines 5 and 9 have been observed to be acetylated on histone H2A. With so many different sites for acetylation, a high level of specificity can be achieved in triggering specific responses. An example of this specificity is when histone H4 is acetylated at lysines 5 and 12. This acetylation pattern has been seen during histone synthesis. Another example is acetylation of H4K16, which has been associated with dosage compensation of the male X chromosome in "Drosophila melanogaster".