It was previously thought a select group of "master" genes was responsible for controlling the growth of cells that can cause the conditions.
But the study discovered that there are actually hundreds of genes that interact with each other.
Scientists said they believed variations in this network explained why people develop diseases in different ways.
The team hopes that by identifying weak spots in the gene structure they will be able to stop the growth of tumours.
Professor David Hume, of the University of Edinburgh's Roslin Institute, said: "This study has effectively shown us where the brakes are which could slow down or stop diseases like cancer and multiple sclerosis.
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