Ginnie Mae, guarantee against homeowner default risk. In the case of Ginnie Mae, this guarantee is backed with the full faith and credit of the US federal government. This is not the case with Fannie Mae and Freddie Mac, but these two entities have lines of credit with the US federal government; however, these lines of credit are extremely small compared to the average amount of money circulated through these entities in one day's business. Additionally, Fannie Mae and Freddie Mac generally require private mortgage insurance on loans in which the borrower provides a down payment that is less than 20% of the property value. Pooling many mortgages with uncorrelated default probabilities creates a bond with a much lower probability of total default, in which no homeowners are able to make their payments (see Copula). Although the risk neutral credit spread is theoretically identical between a mortgage ensemble and the average mortgage within it, the chance of catastrophic loss is reduced. If the property owner should default, the property remains as collateral. Although real estate prices can move below the value of the original loan, this increases the solidity of the payment guarantees and deters borrower default. If the MBS was not underwritten by the original real estate and the issuer's guarantee, the rating of the bonds would be much lower. Part of the reason is the expected adverse selection against borrowers with improving credit (from MBSs pooled by initial credit quality) who would have an incentive to refinance (ultimately joining an MBS pool with a higher credit rating). Real-world pricing Because of the diversity in MBS types, there is wide variety of pricing sources. In general, the more uniform or liquid the MBS, the greater the transparency or availability of prices. Most traders and money managers use Bloomberg and Intex to analyze MBS pools and more esoteric products such as CDOs, although tools such as Citi's The Yield Book, Barclays POINT, and BlackRock's AnSer are also prevalent across Wall Street, especially for multi–asset class managers. Some institutions have also developed their own proprietary software. Tradeweb is used by the largest bond dealers (the "primaries") to transact round lots ($1 million and larger). Complex structured products tend to trade less frequently and involve more negotiation. Prices for these more complicated MBSs, as well as for CMOs and CDOs, tend to be more subjective, often available only from dealers. For "vanilla"