Sometimes new changes (mutations) develop in the part of the BCR-ABL gene that makes the BCR-ABL protein. These mutations change the shape of the BCR-ABL protein, affecting how and which targeted cancer drugs can bind to it to block the growth signals. A mutation analysis is a test that looks for new mutations in the BCR-ABL gene that may occur during treatment for CML. This test may be performed on a peripheral blood or bone marrow sample after months of treatment based on how well treatment is working. Mutational analysis is important because new or different gene mutations can affect which treatment option is best for you.