In 1971, DePatie-Freleng took over the license to adapt the works of Dr. Seuss (Ted Geisel) for television. Their first project was also the studio's first sale to CBS, The Cat in the Hat, which also marked the end of Chuck Jones' association with the franchise.
Jones, who would have a hand in ABC's Curiosity Shop that year, served as co-executive producer with Geisel, David DePatie, and Friz Freleng. Comedian Allan Sherman was cast in the title role of the Cat, and doubled as narrator. Additional voices include Daws Butler and Pamelyn Ferdin (who would also appear on Curiosity Shop). Universal owns the rights now, likely by virtue of having done a live-action version 12 years ago with Mike Myers (ex-Saturday Night Live) as the Cat.
To this day, the Cat and Things 1 & 2 have been used for merchandising. Like, every so often, you'll see someone wearing a Thing 1 or 2 shirt, for example.