Mechanical properties like stiffness often signal cells how to grow, including cells in the breast. Here, flexible scaffolding between cells encourages healthy growth and differentiation during development and reproduction, whereas a relatively stiff framework is linked to abnormalities such as the formation and growth of tumors. But exactly how mechanical integrity is translated into biochemical signals has remained unclear. Now, a close look at breast cells grown in 3D offers some important clues. Researchers grew breast cells on collagen structures of varying stiffness. They found that stiffer structures (containing more collagen) indeed led to irregular cell formations, namely, the malformation of spherical compartments of breast tissue known as acini. They discovered that the molecule partially responsible for abnormal acini growth was MRTF-A, a regulator of smooth muscle formation. But more work is needed to clarify MRTF-A’s role. Because while it did tend to corrupt the inner layers of budding acini, in some cases, MRTF-A stopped abnormal growth of the outer layers. Clarifying MRTF-A’s role and other effects regulated by mechanical stiffness could lead to new perspectives on how breast cancer forms.