Phytochrome is a soluble protein that regulates various responses of plants to light. Not all but most of the phytochrome responses are accompanied by changes in the pattern of gene expression. Upon light activation, phytochrome is imported into the nucleus by the nuclear localization activity of the carboxy-terminal half of the molecule. In darkness, the amino-terminal chromophoric domain suppresses this activity to retain the molecule in the cytoplasm. In the nucleus, light-activated phytochrome forms speckles whose biological function remains unclear.