OCTN2 is consistently expressed at high levels in brain microvessel endothelial cells. Disclosed herein are assays for determining whether a test material/molecule is a substrate for, and/or is actively transported by, the OCTN2 transporter, and therefore a candidate substrate for crossing the blood brain barrier. The assays are useful in screening for therapeutic, cytotoxic or imaging compounds used in the treatment or diagnosis of neurological diseases.