The expression of the ALK gene (as part of a fusion gene or otherwise) can cause a cell to become cancerous through its signaling pathway. As shown below, the ALK enzyme can pass a signal to other kinases (including P13K, Stat3, and Jak2) which in turn transmit a signal downstream through the signal pathway to the nucleus where the expression of different genes are controled (increased or decreased). Thus ALK can cause a cell to grow, split, and survive (which when inappropriately done can start a cancer tumor). ALK inhibitors are drugs that are designed to interfere with the ALK signaling pathway.
See “Inhibition of ALK Signaling for Cancer Therapy” as the source of this diagram.