In this task, you are given a sentence. You are expected to recognize the name of any form of chemical substances. Although there might be several correct answers, you need to write one of them.

Q: Sulfasalazine, a potent suppressor of lymphoma growth by inhibition of the x(c)- cystine transporter: a new action for an old drug.

A: cystine
****
Q: Estradiol replacement enhances cocaine-stimulated locomotion in female C57BL/6 mice through estrogen receptor alpha.

A: estrogen
****
Q: The structures of thymidine kinase from herpes simplex virus type 1 in complex with substrates and a substrate analogue.

A:
thymidine
****