Jamqoi Gyaincain (1302-1364) was an offspring of the Lang clan. With help from Yuan Imperial Tutor Gunggar Lhozui Gyaincain Beisang, he replaced the chief of the Pabmo Zhuba 10,000-Households and won the seal of authority from the Yuan emperor in September 1322, when he was 20 years old.

In 1354, when Jamqoi Gyaincain was 53, he founded the Pagmo Zhuba Disi (Regent) Regime, building Sangzhubsang Castle in present-day Xigaze. In 1358, when he captured Sagya, the Yuan emperor ordered him to take over the affairs of U-Tsang. From then on, the Pagmo Zhuba regime won confirmation from the Yuan Dynasty and had its capital at Nedong.

During his reign, Jamqoi Gyaincain introduced the Zong system by abolishing the 10,000-Households of the Sagyapa Regime and replacing them with 13 Zongs. Each Zong, which originally means castle, was head by a Zongben, who ruled the Zong for a three-year term. The Zong gradually expanded in scope to the level of the present-day county.

Jamqoi Gyaincain also called for the planting of trees. In his History of the Lang Family, he said: "We need to plant 200,000 willow trees a year. Timber thus gained could be used to repair monasteries and build houses."

He also called on the populace to reclaim wasteland.

Thanks to these measures, the regime enjoyed an economic and cultural boom. It was during this period that Tangdong Gyibo (1361-1485) built dozens of iron-chain bridges and created Tibetan opera; Zongkapa founded the Gelug Sect, initiated the Grand Summons Ceremony, and, together with his disciples, built the three major monasteries in Lhasa as well as Tashilhungpo Monastery.