Hexavalent chromium [Cr(VI)] is a well known mutagen and carcinogen. Since 
genomic instability due to generation of double strand breaks (DSBs) is causally 
linked to carcinogenesis, we tested a hypothesis that Cr(VI) causes in vivo 
generation of DSBs and elicits DNA damage response. We fed repair proficient 
Drosophila melanogaster (Oregon R(+)) larvae Cr(VI) (20.0μg/ml) mixed food for 
24 and 48h and observed a significant (p<0.05) induction of DSBs in their midgut 
cells after 48h using neutral Comet assay. Global gene expression profiling in 
Cr(VI)-exposed Oregon R(+) larvae unveiled mis-regulation of DSBs responsive 
repair genes both after 24 and 48h. In vivo generation of DSBs in exposed 
Drosophila was confirmed by an increased pH2Av immunostaining along with the 
activation of cell cycle regulation genes. Analysis of mis-regulated genes 
grouped under DSB response by GOEAST indicated the participation of 
non-homologous end joining (NHEJ) DSB repair pathway. We selected two strains, 
one mutant (ligIV) and another ku80-RNAi (knockdown of ku80), whose functions 
are essentially linked to NHEJ-DSB repair pathway. As a proof of principle, we 
compared the DSBs generation in larvae of these two strains with that of repair 
proficient Oregon R(+). Along with this, DSBs generation in spn-A and okr 
[essential genes in homologous recombination repair (HR) pathway] mutants was 
also tested for the possible involvement of HR-DSB repair. A significantly 
increased DSBs generation in the exposed ku80-RNAi and ligIV (mutant) larvae 
because of impaired repair, concomitant with an insignificant DSBs generation in 
okr and spn-A mutant larvae indicates an active participation of NHEJ repair 
pathway. The study, first of its kind to our knowledge, while providing 
evidences for in vivo generation of DSBs in Cr(VI) exposed Drosophila larvae, 
assumes significance for its relevance to higher organisms due to causal link 
between DSB generation and Cr(VI)-induced carcinogenesis.