In an Advanced Online Publication in Nature Biotechnology, Paul Mintz and colleagues at the University of Texas M.D. Anderson Cancer Center in Texas, describe the use of phage-display technology to examine the repertoire of circulating, anti-tumour antibodies in the blood of prostate cancer patients (Nature Biotechnology, 23 December 2002, DOI:10.1038/nbt774). To characterise the 'fingerprint' of circulating antibodies they screened a phage random-peptide library with purified immunoglobulins from the serum of cancer patients and identified a number of immunoreactive peptide motifs. Reactivity correlated with disease progression and poor clinical prognosis. The anti-peptide antibodies recognise the glucose-regulated protein GRP78 which was also upregulated in metastatic prostate cancer.