The phrase "is responsible for" is a common one used to describe accountability for a particular action or outcome. However, there are several synonyms that can be used in place of this phrase to add variety to your writing. Some of these synonyms include "is to blame for," "is accountable for," "bears the responsibility for," "is in charge of," "is tasked with," and "has the duty of." Using these synonyms can help to convey a different tone or nuance to your writing, and can also make your sentences less repetitive. It's always good to have a variety of words and phrases available to use when writing to avoid sounding boring or monotonous.