Thank Bear...this explains it:

"xB3 is a peptide that binds to the LRP1 receptor. The LRP1 receptor is highly/densely expressed at the BBB. Other technologies, such as those that target the transferrin receptor (TfR), are at a disadvantage because the transferrin receptor is not as highly/densely expressed at the BBB. More LRP1 at the BBB equals more doors. Less TfR at the BBB equals less doors."

Mark Day I suppose could have expanded on this in the interview the way you did. You did it in a couple of sentences. And yes, the video tells the receptor story quite well. I have seen it many times.

Koo