Mice lacking the serotonin transporter (5-HTT) do not exhibit the typical behavioral responses to psilocybin, such as head-twitch and increased locomotion, observed in normal mice. This underscores the importance of 5-HTT in how psilocybin affects behavior, suggesting that similar mechanisms may influence its therapeutic effects in humans, particularly for conditions like depression and anxiety where serotonin pathways play a key role.