Cell signaling is one of the more difficult concepts for students to understand in their study of cell biology. To help students better understand the process of cell signaling, from reception to cellular response, a fairly simple signal pathway is used as an example in this case study in which students read a series of instant messages and emails between two friends trying to determine what is causing erectile dysfunction in a prize winning dog. The case was developed for use in an introductory biology course, but could also be used in a cell biology course.