Here is a link to a article about the science related to the ALK gene and protean and how that fits with the development of ALK inhibitors. It includes a chart on the estimated number of patients who develop different ALK + cancers (lung, lymphoma, neuroblastoma, and Inflammatory myofibroblastic). The estimated number of lung cancer patients is greater than all of the other forms of cancer combined.
Here is a 2008 Nature.com article about chromosome translocations in general. Chromosome translocations are important because they appear to be the main way in which fusion genes are created and ALK-EML4 is a fusion gene. Apparently many fusion genes cause cancers. Also the article suggests that early research suggested the exposure to X-rays greatly increases the likelihood of chromosome translocations. This article does not go into depth on other potential causes of chromosome translocations.