CDO issuance grew from an estimated $20 billion in Q1 2004 to its peak of over $180 billion by Q1 2007, then declined back under $20 billion by Q1 2008. Further, the credit quality of CDO's declined from 2000 to 2007, as the level of subprime and other non-prime mortgage debt increased from 5% to 36% of CDO assets. As described in the section on subprime lending, the CDS and portfolio of CDS called synthetic CDO enabled a theoretically infinite amount to be wagered on the finite value of housing loans outstanding, provided that buyers and sellers of the derivatives could be found. For example, buying a CDS to insure a CDO ended up giving the seller the same risk as if they owned the CDO, when those CDO's became worthless.