The Limbu(exonym) or Yakthung(endonym) () tribes and clans are a Kirati people indigenous to the hill and mountainous regions of East Nepal between the Arun and Mechi rivers to as far as South Tibet, Bhutan, Darjeeling, Sikkim, Assam, Nagaland, Burma and now many migrated to UK, USA, Hongkong China and other countries. The name Limbu is an exonym of an uncertain origin. They call themselves Yakthumba (endonym) (. In Standard Tibetan, they are referred to as “Sikkimese” () and in Sikkimese as Shong or Tsong “Tsang people”). Their estimated population of 700,000 is centered in the districts of Sankhuwasabha, Tehrathum, Dhankuta, Taplejung, Morang, Sunsari, Jhapa, Panchthar, Ilam District, Kathmandu District, Lalitpur District, Bhaktapur District’ in Nepal, These are all within the Mechi and Kosi Zones or “Limbuwan”. Portions of the Limbu population are also located in the east and west districts of Sikkim. A smaller number are scattered throughout the cities of Darjeeling and Kalimpong in West Bengal, Assam, Nagaland and in north and south Sikkim, Bhutan, Burma and recently migrated to UK, Hongkong China, USA and many other countries. Limbu clans and tribes are divided into two gotras indicating their origin: Lhasa and Yunnan. The Limbu are known as Yakthung Thibong () or “Ten Limbu”, from which thirteen Limbu subgroups have emerged. According to legend, five of the groups came from Yunnan and the other eight from Lhasa. Unlike the caste system which was brought to the Nepal region after the invasion of the Hindu Gorkhas, a caste system is not practiced among the Limbu people and Limbu sub-groups. However, there are numerous different clans and family-sects. The Chinese text called Po-ou-Yeo-Jing translated in 308 AD refers to the Yi-ti-Sai (barbarians bordering on the north), a name which is an exact equivalent of Kiratas.