Pound for pound, the most profitable banks and thrifts that are focused on the mortgage market continue to get most of their income from their long-term mortgage investments, rather than less reliable non-interest income. Among depository institutions that have $50 million or more in assets, at least half of which are invested in mortgages or MBSs, Merrill Lynch Trust Co., a Delaware-based federal savings bank, remained the most profitable at the end of 2003.