 Each core histone possesses a 'tail' structure, which can vary in several ways; it is thought that these variations act as "master control switches", which determine the overall arrangement of the chromatin.In particular, it is believed that the presence of methylated lysine 4 on the histone tails acts as a general marker.In general, euchromatin appears as light-colored bands when stained in G banding and observed under an optical microscope, in contrast to heterochromatin, which stains darkly.