CA 15-3 and Related Mucins as Circulating Markers in Breast Cancer

In many Western countries breast cancer is the most common malignancy affecting women, the major cause of cancer deaths in women and the major cause of death from any cause in women aged 35-54 years.' By the year 2000, it is estimated that approximately 1 million new cases per year will be diagnosed worldwide. This will represent an increase of 24% for the period 1990--2000, or over 2% per year.' Because of its enormous impact on society, more funding has probably been devoted to research into breast cancer than into any other malignancy. One of the key aims of this research has been to develop biochemical tests for aiding diagnosis, assessing prognosis, predicting likely response to therapy and monitoring patients. Amongst the clinically most useful tests to result from this work have been the assays of oestrogen and progesterone receptors (ER and PR). For over 20 years, tissue-based assays of these two receptors have been in routine use both for predicting hormone dependence of breast cancers and for assessing prognosis.? In addition to ER and PR, many other tissue-based assays have been investigated for use in aiding prognosis. These include the genes and their protein products directly involved in carcinogenesis (e.g. c-oncogenes and tumour suppression genes) and metastasis [e.g. proteases such as urokinase plasminogen activator and cathepsin D (for reviews see references 3 and 4)]. Although some of these new markers (e.g. urokinase plasminogen activator) provide prognostic information that is independent of the traditionally used parameters such as tumour size and nodal status, none is yet in routine clinical use. In contrast to the moderate success with tissue-based markers, attempts to find a sensitive and specific circulating breast cancer marker