In one aspect, provided herein are methods for treating cancer in a subject, comprising: (1) identifying in the subject the presence of a mutation in a splicing factor selected from the group consisting of U2AF1, SF3B1, SRSF2, and ZRSR2 and/or determining in the subject an increased amount of DCAF15 compared to a control, and (2) inhibiting an activity of RBM39 in the subject. In some embodiments, the inhibiting step can include promoting RBM39 degradation, preferably in a DCAF15-dependent manner. Compositions are also provided.