The question of whether regulators should curb Fannie Mae’s and Freddie Mac’s investments in mortgage-backed securities has been hotly debated since Federal Reserve Chairman Alan Greenspan recommended such limits in February. As for the Federal Home Loan Banks, however, this issue has already been resolved. The Federal Housing Finance Board links the amount of MBS a particular FHLBank can hold to its total capital, and prohibits any Bank from additional purchases if its portfolio exceeds 300