Oak Brook-based Portillo Restaurant Group Inc. confirmed Tuesday afternoon that it has agreed to sell its restaurant chain to Berkshire Partners, a private equity firm in Boston.

Terms of the deal were not disclosed, but some reports say it could be about $1 billion.

Berkshire Partners said in a statement that it will work closely with Dick Portillo, founder of Portillo's, to support the growth of the company.

From a small trailer in Villa Park in 1963, Portillo's has grown to average more than $8 million in sales per store annually.

Portillo's is famous for its Chicago-style hot dogs and Italian beef sandwiches and the creative décor of its dining rooms.

Under Portillo's leadership, Portillo's has experienced significant growth over the past 50 years and currently operates 38 locations across Illinois, Indiana, California and Arizona.

Two new locations opened in Arizona just last year.

"Portillo's is my life's work and I remain committed to ensuring the continued growth and success of the business," Portillo said in a statement. "I was seeking an experienced partner that shared our vision for the company and an appreciation for our culture. After careful consideration, Berkshire Partners proved to be the best choice. I look forward to working together to further strengthen our company."

Josh Lutzker, managing director at Berkshire Partners, said, "Portillo's is a highly differentiated restaurant with a passionate customer base. In addition to great food, the Portillo's name stands for quality and service. We are looking forward to working with Dick and his management team to uphold their culture of excellence. We see great potential in the business and share Dick's vision to methodically expand operations in Chicago and beyond."