The discovery of endoreduplication in the majority of cells of the thorax of Drosophila has implications for genomics, transcriptome levels, chromatin structure and life history of these model insects. The ratio of 2C/4C DNA amounts is 2.00 for nuclei from the head, yet is 1.75 and 1.83 for nuclei from the thorax of wild type and suppressor of underreplication (SuUR) strains, respectively. The latter ratios reflect underreplication in the majority of nuclei from the thorax, which is only partially suppressed in the SuUR strain. The effect is age-dependent. Thoracic 4C DNA is significantly more underreplicated in the nuclei of 10 day old than newly emerged flies. The consequences of underreplication for the majority of thoracic cell nuclei, likely mimic those in highly endoreduplicated polytene salivary and nurse cell nuclei, which would affect expression levels in genomic, transcriptomic, methalomic and other studies based wholly or in part on Drosophila thoracic nuclei. .Overexpression of the SuUR gene induces reversible modifications at pericentric, telomeric and intercalary heterochromatin of Drosophila melanogaster polytene chromosomes.