X0101 EFFECT 33 LIVE 2012-11-02 0001144204-12-054375 485APOS 0001469192 MAINSTAY FUNDS TRUST 333-160918 S000039358 MainStay Cornerstone Growth Fund C000121293 Class A S000039358 MainStay Cornerstone Growth Fund C000121294 Class B S000039358 MainStay Cornerstone Growth Fund C000121295 Class C S000039358 MainStay Cornerstone Growth Fund C000121296 Class I S000039358 MainStay Cornerstone Growth Fund C000121297 Class R2 S000039358 MainStay Cornerstone Growth Fund C000121298 Investor Class