After a glance at Berkshire Hathaway's public equities portfolio, you'll quickly realize that just a handful of businesses dominate. I'm talking about Apple, Bank of America, and American Express, which -- combined -- constitute an astounding 63.8% of its holdings.But there are also little-known positions like Sirius XM Holdings (SIRI -1.80%). This audio entertainment stock makes up a tiny portion of the overall portfolio and has fallen 15% in the last five years.Despite the Oracle of Omaha and his team's s ...