A popular exchange-traded fund that tracks the health-care sector saw its best daily return in 10 months on Friday after the Trump administration proposed new rules aimed at making health-care prices more transparent. The SPDR Health Care Select Sector ETF /zigman2/quotes/205918244/composite XLV -0.44% posted a 2.1%, marking its best day since Jan. 4, when it rose 2.98%, according to FactSet data. The Trump administration proposed a rule that would require health insurers to disclose estimated out-of-pocket costs, including negotiated rates, to patients in advance of a procedure or office visit. It also finalized a rule requiring hospitals to make transparent the prices they charge patients starting in 2021. It's possible that the stocks gained on doubts on how the rules would be enforced. "While the Trump administration's new hospital price transparency requirement is quite sweeping, the enforcement of it is quite weak - a maximum fine of $300 per day," Kaiser Family Foundation executive Larry Levitt wrote on Twitter . Shares of UnitedHealth Group Inc. /zigman2/quotes/210453738/composite UNH +0.29% , which is the largest health insurer in the U.S., and Anthem Inc. /zigman2/quotes/203808743/composite ANTM +0.05% each rose roughly 5%. Shares of Tenet Healthcare Corp. /zigman2/quotes/207964487/composite THC +0.68% and HCA Healthcare Inc. /zigman2/quotes/205934574/composite HCA +2.00% , both large hospital operators, were up at least 2%. The S&P 500 /zigman2/quotes/210599714/realtime SPX +0.36% is up 24.4% year-to-date.