LC3B (Microtubule-associated protein light chain 3B) is the most widely-studied isoform within LC3/GABARAP autophagy family. Soluble LC3B-I resides in cytosol; lipid-modified LC3B-II associates with autophagosome membrane to support autophagosome assembly and serves as autophagosome reporter. Autophagy activation elevates LC3B-II levels, yet autophagic-flux blockage also causes LC3B-II accumulation; flux assays with lysosomal inhibitors are required for correct interpretation.