Lewis Ranieri, once known as the father of mortgage finance, is daring to revisit the most infamous sector of the mortgage market - sub-prime lending.

Four years ago, sky-high defaults on sub-prime mortgages - used mainly by borrowers with weak credit profiles - touched off the global financial meltdown, wiped out hundreds of financial institutions worldwide and destroyed billions of dollars invested in mortgage-backed securities. Today, lawsuits against Wall Street firms from investors burned by souring mortgage securities continue to meander through the courts.