WASHINGTON ( TheStreet) -- It's easy to define which mortgage lenders will be affected by the wind-down and replacement of Fannie Mae ( FNMA.OB) and Freddie Mac ( FMCC.OB): All of them.

It's harder to figure out which ones will be affected the most - and nearly impossible to tell whether the effect will be a good one or bad one until Congress decides how it will overhaul the entire housing-finance system.

There are concerns that holders of so-called "agency" mortgage-backed securities (MBS) -- those notes issued by Fannie and Freddie -- could be left holding "orphans" if the securities are replaced by something new. On the other hand, the market for agency MBS is so huge and liquid that there's little chance investors wouldn't be able to buy or sell the securities if they needed to. Additionally, holding that high-quality mortgage debt to maturity, receiving reliable payments, isn't such a bad thing for fixed-income investors.

In any case, it's too early to say what will become of the U.S. housing finance system but change is a sure thing. To get an idea of which banks have the most skin in the Fannie-Freddie game, TheStreet examined exposure to agency MBS, relative to total assets. Those are mortgage bonds - as opposed to whole loans - guaranteed by the government-sponsored enterprises (GSEs) Fannie Mae and Freddie Mac or the outright government agency Ginnie Mae.

It's not perfect science, but it gives a sense of which banks are loaded up with agency MBS and will have to be most nimble to upcoming changes.

The data come from 83 banks' fourth-quarter filings with the Federal Reserve, via SNL Financial. Not all banks have reported their quarterly information yet, while others don't disclose the necessary level of detail, so the list is not based on a comprehensive examination of the industry.

However, it's worth noting that the banks with the most nominal exposure to agency MBS don't have the most exposure relative to their balance sheets. The banks with the most agency MBS are Bank of America ( BAC), Wells Fargo ( WFC) and JPMorgan Chase ( JPM), with a combined $429.2 billion. But agency MBS comprise just 6% to 10% relative to their massive, diverse balance sheets.

The average of the 83 banks whose data were available is 14.1% - meaning BofA, Wells and JPMorgan have less exposure than most. By contrast, the bank with the most exposure has more than 58% of its balance sheet tied up in agency MBS.

On the following pages are the 10 banks with the most exposure to agency MBS, based on the information currently available:

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