The present invention generally relates to the fields of 
cancer therapy and 
cancer prevention. More particularly, the present invention generally relates to a 
diagnostic marker for predicting the 
efficacy of 
topoisomerase I (topo I) inhibitors in the treatment of cancers. More specifically, the present invention relates to methods, machines, computer systems, computable readable media and kits which can be used to identify and determine the effectiveness of 
topoisomerase I (topo I) inhibitors in the treatment of cancers, and in some embodiments, the level of sensitivity or resistance of a tumor 
cell to a 
topoisomerase I inhibitor, such as 
camptothecin (CPT), or CTP analogues such as 
topotecan and 
irinotecan and derivatives thereof. More specifically, the present invention related to methods, machines, computer systems, computable readable media and kits which can be used to determine the presence of 
phosphorylation of topoisomerase I polypeptide, in some embodiments 
phosphorylation at residue 
serine 10 (S10) of a topoisomerase I polypeptide, wherein the presence of 
phosphorylation, in particular the phosphorylation at 
serine 10 of a topoI polypeptide indicates a 
cancer is likely to be unresponsive to a topo I inhibitor, whereas the absence of phosphorylation, in particular, the absence of phosphorylation at residue 
serine 10 (S10) identifies a cancer is likely to be responsive to a topo I inhibitor. Other aspect of the present invention relate to phospho-serine 10 topoisomerase I antibodies and other 
protein binding moieties, and uses thereof.