There is great interest in the detection of circulating tumour cells (CTCs) as an important diagnostic and prognostic indicator for patients with many (if not all) types of cancer, and many studies have established that the absolute level of CTCs is a critical determinant. Given that, most studies in the field now utilise reverse transcription/polymerase chain reaction-based measurements, focussing on selected marker RNAs for the particular tumour type. However, such measurements mandate choosing the marker RNAs in advance, and only a limited number of markers can be examined in the reactions. Clearly, a more robust assay would allow simultaneous measurement of many different RNAs, and the ability to look for many different types of cancer would provide a major advantage as a potential screening tool.