In ep 8 ("And the Sky Full of Stars") we see a Minbari holding up a little triangle in front of the captured Sinclair, and it starts glowing, freaking the Minbari hard enough to stop the war then and there. It's not until ep 75 ("Atonement", IIRC) that we find out that the triangle ("Triluminary") is reacting to the presence of Valen's DNA.