If you're in the market for a teacup kitten, remember it's buyer beware. Unscrupulous breeders mate dwarf cats or runts of the litter to produce smaller felines, with little regard to potential health issues. Certain Persian cat fanciers are developing a "teacup" line by breeding animals with a genetic propensity for small size, without serious health problems. That's the sort of breeder you should contact if you want to purchase a teacup kitten.
In Persian cat lines, felines of specific shades tend to be smaller than those of other colors. Since shaded silver or gold Persians run smaller than the average member of the breed, it's likely any kitten marketed as "toy," mini" or "teacup" is one of those shades. As of 2014, the breeder can register these kittens as Persians with the Cat Fanciers Association or other reputable breed organizations, but there's no breed standard or recognition for the teacup. While it's likely such kittens will mature as smaller than average, there's no real guarantee. Before buying a teacup kitten, make sure you won't end up disappointed if he grows larger than expected.
If you want to ensure a cat stays small, a better choice might be the Singapura. While not marketed as a teacup, these friendly Asian cats don't weigh more than 8 pounds at maturity, with females usually weighing a few pounds less than males. They often don't reach their mature size until the age of 2.