MYC overexpression is thought to initiate tumorigenesis by inducing cellular 
proliferation and growth and to be restrained from causing tumorigenesis by 
inducing cell cycle arrest, cellular senescence, and/or apoptosis. Here we show 
that MYC can induce DNA breaks both in vitro and in vivo independent of 
increased production of reactive oxygen species (ROS). We provide an insight 
into the specific circumstances under which MYC generates ROS in vitro and 
propose a possible mechanism. We found that MYC induces DNA double-strand breaks 
(DSBs) independent of ROS production in murine lymphocytes in vivo as well as in 
normal human foreskin fibroblasts (NHFs) in vitro in normal (10%) serum, as 
measured by gammaH2AX staining. However, NHFs cultured in vitro in low serum 
(0.05%) and/or ambient oxygen saturation resulted in ROS-associated oxidative 
damage and DNA single-strand breaks (SSBs), as measured by Ape-1 staining. In 
NHFs cultured in low versus normal serum, MYC induced increased expression of 
CYP2C9, a gene product well known to be associated with ROS production. Specific 
inhibition of CYP2C9 by small interfering RNA was shown to partially inhibit 
MYC-induced ROS production. Hence, MYC overexpression can induce ROS and SSBs 
under some conditions, but generally induces widespread DSBs in vivo and in 
vitro independent of ROS production.