Histones and their posttranslational modifications influence the regulation of many DNA-dependent processes. Although an essential role for histone-modifying enzymes in these processes is well established, defining the specific contribution of individual histone residues remains a challenge because many histone-modifying enzymes have nonhistone targets. This challenge is exacerbated by the paucity of suitable approaches to genetically engineer histone genes in metazoans. Here, we describe a platform in Drosophila for generating and analyzing any desired histone genotype, and we use it to test the in vivo function of three histone residues. We demonstrate that H4K20 is neither essential for DNA replication nor for completion of development, unlike inferences drawn from analyses of H4K20 methyltransferases. We also show that H3K36 is required for viability and H3K27 is essential for maintenance of cellular identity but not for gene activation. These findings highlight the power of engineering histones to interrogate genome structure and function in animals. Display omitted •Specific histone genotypes can be engineered in Drosophila using BAC transgenes•Histone gene expression is controlled by an active dosage compensation mechanism•Posttranslational modification of H4K20 is not required to complete development•H3K27 is required for Polycomb target gene repression but not for gene activation McKay et al. describe a platform for histone gene engineering in Drosophila that allows direct genetic assessment of histone residue function in vivo. Using the system, they demonstrate that, contrary to inferences based on analysis of H4K20 methyltransferases, H4K20 is not essential for DNA replication or completion of development.