Since the 1920s, girdling of Thompson Seedless vines has been used to increase the size of the table grapes. Research on growth regulators started in 1949 when 31 different hormones were tested on Black Corinth and Thompson Seedless grapes. The two most promising regulators of the auxin type (a plant growth hormone regulating cell division) were 4-chlorophenoxyacetic acid (4-CPA) and benzothiazole-2-oxyacetic acid (BOA). For several years 4-CPA was used commercially, but it is no longer registered for use on grapes; BOA has never been used commercially.