def is_homozygous(self):
        '''Returns true iff this variant has a GT field and is homozygous, which here
        means that the genotype is n/n (where n can be any number).'''
        if self.FORMAT is None:
            return False
        else:
            genotypes = set(self.FORMAT.get('GT', '0/1').split('/'))
            return '.' not in genotypes and len(genotypes) == 1