In Section 3.3 below, the engineering of BBB-penetrating BSAs will be reviewed for therapeutic antibodies that target Abeta amyloid, beta secretase 1 (BACE1), or triggering receptor for myeloid cells 2 (TREM2), for the treatment of Alzheimer’s disease (AD), and therapeutic antibodies that target α-synuclein (SYN), or tropomyosin receptor kinase B (trkB), for the treatment of Parkinson’s disease (PD). Here, BACE1 is linked to early-onset autosomal dominant Alzheimer disease.