Guided by our findings from immuno-MRM-MS that pRAD50 may have better dynamic range than pATM when interrogating ATM pathway activation, and supporting published work that RAD50 is phosphorylated directly by ATM at Ser635 following DNA damage,30 we developed an IHC assay to assess modulation of ATM signalling in tumour tissue using the CST #14223 pRAD50(Ser635) antibody.33 Here we report the full development, validation and broader applicability of the pRAD50 IHC assay. Here, RAD50 is linked to neoplasm.