Redwood Trust issued its second non-agency MBS of the year this week with just one rating, a sign of dramatic change in the role of credit ratings in the market. During the heyday of the non-agency MBS market, very few public deals went to market without at least two ratings and some transactions were rated by all three of the top credit rating services. Standard & Poor’s was the market leader for years, but Fitch has had a virtual monopoly on the jumbo MBS sector, which totals just two deals so far this year. S&P has effectively taken to the sidelines in rating non-agency MBS backed by new mortgages, according to...