One of the most powerful ways of studying the function of a protein is to specifically block its activity within cells. Over the past decade, dominant-inhibitory proteins have emerged as popular tools with which to accomplish this task; these mutated proteins interfere with the function of their normal cellular counterparts or with proteins that interact with them. This approach has been used extensively in the elucidation of signal-transduction cascades, such as those involving Ras-family proteins. Here I discuss the power and potential pitfalls of using dominant-inhibitory Ras proteins.