The general principles of signal transduction can by compared to air traffic between different countries, where countries symbolize the cells and airplanes carrying passengers or cargo symbolize the signal-transmitting proteins. These proteins bind to membrane-anchored receptors, which transmit the signal via the membrane into the cell. Within the cell, the signal is transmitted between cytosolic proteins. These proteins can modulate the signal and transmit it to the cell nucleus. Here the biological response to the original signal is e.g. changing the expression level of one or more distinct genes.