Provided herein are methods of treating a patient having a cancer that exhibits (i) a hemizygous loss of the TP53 gene; (ii) a hemizygous loss of the POLR2A gene; and/or (iii) a decreased level of expression of a POLR2A gene product relative to a reference (i.e., control) expression level. The methods comprise administering a therapeutically effective amount of a POLR2A inhibitor (e.g., a nucleic acid that inhibits the expression of a POLR2A protein, an amatoxin, alpha-amanitin, or alpha-amanitin conjugated to a cell targeting moiety, such as an EpCAM antibody) to a patient having or determined to have (i) a hemizygous loss of the TP53 gene; (ii) a hemizygous loss of the POLR2A gene; and/or (iii) a decreased level of expression of a POLR2A gene product relative to a reference (i.e., control) level.