Biological systems, including cells, tissues and organs, can function properly only when their parts are working in harmony. These systems are often dauntingly complex: Inside a single cell, thousands of proteins interact with each other to determine how the cell will develop and respond to its environment.

To understand this great complexity, a growing number of biologists and bioengineers are turning to computational models. This approach, known as systems biology, has been used successfully to model the behavior of cells grown in laboratory dishes. However, until now, no one has used it to model the behavior of cells inside a living animal.

via In vivo systems biology.