Sam Zell, the billionaire businessman who founded Equity Group Investments (EGI), died Thursday at his home due to complications from a recent illness, the company announced. He was 81.

Among his many ventures, Zell will be remembered by the real estate community for creating what’s now known as the modern real estate investment trust (REIT).

Zell was born to Polish refugees in Chicago in 1941. The family later relocated to the suburbs, where Zell attended Highland Park High School before going on to the University of Michigan. While there, he began his real estate career by managing student housing. Five years after graduating, he founded EGI: a private investment firm headquartered in his hometown.

Alongside his business partner and former fraternity brother, Bob Lurie, Zell built EGI into an empire, branching out from real estate to invest across various enterprises throughout the 1970s and ’80s.

Shortly after Lurie’s death in 1990, Zell launched what became some of the largest REITs in real estate history: Equity Residential, an apartment REIT; Equity LifeStyle Properties, a manufactured home community and resort REIT and Equity Office Properties Trust, which sold for $39 billion in 1997 — the largest leveraged buyout in history at that time. More recently, in 2014, Zell took control of CommonWealth REIT, renaming the company Equity Commonwealth and becoming chairman of the board.

“Sam lived life testing his limits,” his son-in-law Scott Peppet said. “He was a self-made entrepreneur, an industry creator and leader, a brilliant dealmaker, a generous philanthropist and the head of a family he fiercely loved and protected.”

Zell is survived by his wife, two sisters, three children and nine grandchildren.