Multifunctional enzyme acting as 1,4-alpha-D-glucan:1,4-alpha-D-glucan 4-alpha-D-glycosyltransferase and amylo-1,6-glucosidase in glycogen degradation. Transfers a segment of a (1->4)-alpha-D-glucan to a new position in an acceptor, which may be glucose or a (1->4)-alpha-D-glucan. Hydrolysis of (1->6)-alpha-D-glucosidic branch linkages in glycogen phosphorylase limit dextrin. Monomer. Interacts with NHLRC1/malin. Under glycogenolytic conditions localizes to the nucleus. Liver, kidney and lymphoblastoid cells express predominantly isoform 1; whereas muscle and heart express not only isoform 1, but also muscle-specific isoform mRNAs (isoforms 2, 3 and 4). Isoforms 5 and 6 are present in both liver and muscle. The N-terminus is blocked. Ubiquitinated. The disease is caused by variants affecting the gene represented in this entry. The products of the mRNAs termed isoforms 1 to 4 are identical. Belongs to the glycogen debranching enzyme family.