Facultative heterochromatin is a type of chromatin that can switch between a condensed, inactive state and a more relaxed, active state. This flexibility allows certain genes to be turned on or off depending on the cell's needs or environmental conditions. It plays a crucial role in regulating gene expression and maintaining cellular identity. Unlike constitutive heterochromatin, which remains permanently condensed, facultative heterochromatin can change its structure. This dynamic nature is important for processes such as development, differentiation, and response to stress. Examples of facultative heterochromatin include regions of the X chromosome in female mammals, which undergoes inactivation to balance gene dosage between sexes.