They actually just multiply in the same way as normal cells.
How it starts is, there is a problem with the DNA of the cell: either it is damaged or there has been a mistake made in it when it was copied. The genes that are responsible for cell growth have been disrupted, and the new cell just doesn’t look right. It is just round and looks like a blob compared to all the intricate detail of the cells around is. Because the growing genes are damaged, the cancerous cell splits and multiplies like crazy – completely out of control. They group together and form tumours. The tumours have a really high concentration of chemicals called growth factors. These leak into the persons bloodstream and the blood test that I am trying to make is designed to detect them.
Cancer cells can break off from the tumour and migrate to another part of the body and start a new tumour there.