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

The antitussive activity of delta-opioid receptor stimulation in guinea pigs.
delta-opioid receptor

Investigating the enteroenteric recirculation of apixaban, a factor Xa inhibitor: administration of activated charcoal to bile duct-cannulated rats and dogs receiving an intravenous dose and use of drug transporter knockout rats.
factor Xa

Chemical genetic analyses of quantitative changes in Cdk1 activity during the human cell cycle.

Cdk1