Many signaling pathways ultimately pass messages to the nucleus of a cell. The
Raf protein (shown in white) activates another messenger protein (in brown) as
it passes through fibers that make up the cell's cytoskeleton. The signal is
passed to yet another messenger (in purple). These messenger proteins are known
as kinases, enzymes with the ability to activate other proteins through the addition
of phosphate groups. This protein travels to the nucleus past cellular organelles
such as the mitochondria (in glowing orange) and the network of membranes known
as the endoplasmic reticulum (shown in light brown).