The phosphorylation of H2Ax on its S139 site, γH2Ax, is important during DNA double-strand repair and is considered necessary for assembly of repair complexes, but its functional role after other kinds of DNA damage is less clear. We have measured the survival of isogenic mouse cell lines with the H2Ax gene knocked out, and replaced with wild-type or mutant (S139A) H2Ax genes, exposed to a range of agents with varied mechanisms of DNA damage. Knockout and mutant cells were sensitive to γ-rays, etoposide, temozolamide, and endogenously generated reactive oxygen species, each of which can include double-strand breaks among their spectra of DNA lesions. The absence or mutation of H2Ax had no influence on sensitivity to cisplatin or mitomycin C. Although UV light induced the highest levels of γH2Ax, mutation of S139 had no influence on UV sensitivity or the UV DNA damage response. Complete loss of H2Ax reduced the survival of cells exposed to UV light and reduced pChk1 induction, suggesting that sites other than S139 may impact the ATR-pChk1 pathway. The relative intensity of γH2Ax measured in Western blots in wild-type cells did not correlate with the functional importance of γH2Ax. The use of γH2Ax as a general biomarker of DNA damage is therefore potentially misleading because it is not an unambiguous indicator of double-strand breaks, and a significant fraction of DNA repair, especially involving nucleotide excision or crosslink repair, can occur without functional involvement of γH2Ax.