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.
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Question:  Indeed , after a single oral dose , the role of the mu - receptor agonist component of the antinociceptive effect of tramadol appears to be minor , with most of the analgesic effect being attributable to nonopioid properties of the parent compound .

Answer: mu - receptor


Question:  This study was undertaken to evaluate the physical and biological characteristics of nebulized interleukin 2 liposomes .

Answer: interleukin 2


Question:  Thus , transgene expression directed by both the human and mouse Rb promoters is restricted to a subset of tissues in which Rb is normally expressed during embryogenesis .

Answer:
mouse Rb promoters