This region is traditionally defined as the states east of the Mississippi River. The first two tiers of states west of the Mississippi have traditionally been considered part of the West, but can be included in the East today; usually in regional models that exclude a Central region. As of July 1, 2007, the estimated population of the 26 states east of the Mississippi (not including the small portions of Minnesota and Louisiana that are east of the river) plus the District of Columbia totals 171,222,291 out of 305,986,357 in the whole nation (including Puerto Rico and insular areas in the US but not in the East), or 55.95% of the U.S. population. The East no longer has a unified culture, due to the vast number of immigrants who flooded the region from the mid-19th century to the present day. The East is composed of three sub-regions: The South, The Midwest, and The Northeast.