Think of it as though some cellular proteins are painstakingly made by hand while others are mass-produced on an assembly line. Then some cancerous businessman comes in and finds a way to slip the blueprints for the handmade protein into the assembly line. Suddenly the shoddily made copies of the rare protein flood the market using the machinery of a simpler (gene) product.
In real life the assembly line protein is some important part of the immune system, like a piece of the antibody molecule of which we need to be able to produce immense quantities to fight infections. Then a mutation swaps-in a normally highly regulated molecule that controls cell growth and division, like cyclin D. The growth protein is then made at levels reserved for antibodies causing cells to divide in the uncontrolled manner of cancer.The image to the right (from Wikipedia, amazingly) is a nice graphic showing the locations of the chromosomal fragments that swap, as connected by the disease that the swap causes. Notice that a bunch of diseases are connected to chromosome 14; this is where the immunoglobulin heavy chain (IgH), a part of all antibodies, is normally encoded. A translocation between part of chromosome 8 and this region of chromosome 14, or put more succinctly t(8;14), causes Burkitt's lymphoma. t(11;14) causes mantle cell lymphoma and t(14;18) causes follicular lymphoma.
The t(9:22) translocation is particularly interesting. This is called the Philadelphia chromosome (after the city of its discovery) and it's associated with chronic myelogenous leukemia. Here, the gene that's swapped in is not just expressed more but, due to some convenient splicing, actually made more potent. The gene product is a tyrosine kinase, an enzyme that phosphorylates other proteins, and because of this translocation it is constitutively turned on, having widespread and, it turns out, cancerous results.
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