In order to respond to a signal, a cell needs a receptor molecule that recognizes the signal. A cell’s response to a specific signal varies according to the signal. Some signals are local signals (for example, growth factors), while others act as distance signals (for example, hormones). The two basic types of hormone are those that bind to receptors on the cell surface and those whose receptors are found within cytoplasm. Both types can cause the cellular machinery to change its activities.