The whistle register is the range of vocal notes above E6. The ability to hit notes in this register is extremely rare.
A few recording artists who have exhibited the ability to hit notes in the whistle register are Minnie Riperton, Yma Sumac, Shanice Wilson, Chante Moore, Rachelle Ferrell, and Sabrina Johnston. However, the most notable whistle register singer by far is Mariah Carey. While most singers' whistle registers are very faint and unsustainable, Carey has built her career singing in it. Her vocal performance in the song "Bliss" is performed entirely in this register. The Guinness Book of World Records awarded Carey the record for highest note hit by a human in their March 2003 edition for hitting the G7# note during a live performance of "The Star-Spangled Banner". This note was previously considered beyond human range. Incidentally, Mariah Carey also holds the record for largest vocal range; she can hit a note as low as A2.
Though this ability has been most thoroughly recorded in females, it has been reported to occur in males, but it is much more rare.
