There are trillions of cells in the human body but a change in only one can lead to disease.

Certain genes are switched on when the cell becomes cancerous, allowing it to divide rapidly and grow out of control.

Coloured dyes that label these harmful genes could lead to new diagnostic tests.

A team at Albert Einstein College of Medicine in New York, US, made 11 dyes that stick to activated genes within cancerous cells.

Fishing for genes

The technique, known as Fish (Fluorescence in situ hybridization) & Chips, allows researchers to look inside a single cell to see which genes are active.

This level of sensitivity looks set to provide valuable information about the earliest stages of diseases like cancer

Nicola Hawe, Cancer Research UK

It is based on molecular "probes" that bind to DNA within the nucleus of the cell.

When a specific gene is switched-on, the marker triggers a fluorescent signal.

Computer analysis of the resulting multi-coloured pattern can pick up when and where different genes are working.

This could be useful for examining tissue biopsy samples to locate genes involved in tumours, say cancer experts.

'More sensitive'

Advances in gene chip technology have allowed researchers to look at how gene expression patterns within groups of cells relate to healthy and diseased tissues, says Nicola Hawe, Science Information Officer at the charity Cancer Research UK.

"The team led by Jeffrey Levsky have moved the technology a step further and developed a more sensitive method for looking at gene expression in individual cells," she told BBC News Online.

"This level of sensitivity looks set to provide valuable information about the earliest stages of diseases like cancer, where subtle genetic changes in individual cells can sow the first seeds of tumour development."

Full details of the research are published in the journal Science.