Complex sentence: The term "nerve growth factor" usually refers to the 2.5S, 26-kDa beta subunit of the protein, the only component of the 7S NGF complex that is biologically active (i.e. acting as signaling molecules).
Split into simpler sentences: The term "Nerve Growth Factor" usually refers to the 2.5S, 26 kDa beta subunit of the protein. The beta subunit is the only component of the 7S NGF complex that is biologically active (i.e. acting as signaling molecules).