The mortgage-backed securities market is heading for its best first quarter of issuance since 2007. With the launch in the middle of last week of the Resimac Triomphe Trust - Resimac Premier Series 2013-1 - transaction, year to date issuance for 2013 is approaching A$6.5 billion. In the first quarter of 2007, prime RMBS issuance totalled A$13.5 billion.So, RMBS issuance in the first quarter of 2013 is off the 2007 pace but it is far in front of first quarter issuance in any other year since then. The only explanation being offered by the market for this change in investor appetite is that investors are recognising the exceptional value offered by RMBS relative to corporate bonds, where credit spreads have been steadily contracting.A rough comparison shows that 'AAAsf'-rated prime RMBS with a weighted average life of three years can still yield a spread of as much as 120 basis points over bank bills, as the Columbus Triton Trust No.2 Bond Series 2013-1 transaction, priced last Monday, demonstrates. Three-year floating rate notes issued by one of the major banks currently offer a spread of little more than 60 bps.The year 2007 went on to become the second best year for prime RMBS issuance, with A$41.9 billion of notes issued; 2006 was the best year, at A$46.6 billion.But, for the moment, 2011 is the year for 2013 to beat. Prime RMBS issuance totalled A$22.1 billion in 2011.