In 1952, a sire in an artificial insemination (AI) unit in the US was a carrier of red coat colour. Although the AI unit reported the condition and advised breeders as to its mode of inheritance, almost a third of the breeding unit's Holstein inseminations that year were to that red-carrier bull. That year, American AI units had used 67 red-factor bulls that had sired 8250 registered progeny. In spite of this, any change to the colour marking rules was rejected.