<unk>  is the trivial name for 4,4'-diaminobiphenyl, a carcinogenic aromatic amine which has been used as part of a test for cyanide and also in the synthesis of dyes. It has been linked to bladder cancer and pancreatic cancer.
In common with<unk>  some other aromatic amines such as 2-aminonaphthalene have been withdrawn from use in almost all industries because they are so carcinogenic.
In the past a common test for blood used<unk>  but this has largely been replaced by tests using phenolphthalein / hydrogen peroxide and luminol. An enzyme in blood causes the<unk>  to be oxidized to a polymer which is blue coloured. The test for cyanide uses similar chemistry to give the blue colour.<unk>  is a known human carcinogen.
Benzidine
Benzocaine 
Phencyclidine
Methamphetamine
Benzphetamine
Amphetamine
Nicotine 
Nicotine