ALK and anaplastic large cell lymphoma: The ETP stage still has the potential to differentiate into B cell, myeloid, natural killer (NK), and dendritic cell (DC) lineages, and the close relationship of ALK+ ALCL to this stage might explain why myeloid antigen expression is frequently observed in ALK+ ALCL, thereby confusing an ALCL diagnosis (Popnikolov et al., 2000).