Institutional shareholders are reviewing their equity exposures to Commonwealth Bank amid deepening concern that the valuation premium on the company's stock will continue to erode.CBA scrip underperformed all other banking stocks and the wider market on Monday, sliding 68 cents to A$78.41 - its lowest closing price in more than eight months.Since AUSTRAC launched civil proceedings over money laundering matters against the bank on 4 August, the share price has shed $5.56 or almost seven per cent.Many of Australia's largest super funds and listed equity trusts - including Australian Foundation Investment Company, Argo Investments and Australian Super - count CBA as their largest single exposure in their portfolios.Index managers are particularly exposed to the bank's share price weakness because the bank accounts for almost ten per cent of the S&P/ASX 200.While the sustained share price decline is worrying investors, the depth of market concern is reflected in the fact that daily turnover of the stock has been unusually large in the last three weeks.On nine of the twelve trading days since the AUSTRAC litigation was launched in the Federal Court, the volume of CBA scrip traded has exceeded the 12-month daily average.That is a stark contrast to the pattern of trading for Westpac, NAB and ANZ, which have each recorded volumes well below average levels throughout August.It appears that institutional investors are already moving to reweight their exposures to the sector.Jack Lowenstein, chief investment officer at Morphic Asset Management, believes that the slide in market support for CBA's share price might have some way to run."The problem with CBA is that it seems to be driven by key performance indicators across its businesses and if you have a series of knocks to the reputation of the bank, then people within the organisation might start to become uncertain about culture they're working in," he said."In the United States for many years Wells Fargo traded at a premium to other retail banks but that came undone when its sales incentives programs led the organisation into a raft of public scandals."People now looking at the CBA will be fearful that a Wells Fargo scenario might be unfolding in Australia."Peter Rice, the chief investment officer at Pokfulam Investments, expects CBA's share price to remain under pressure until a new chief executive is found to replace Ian Narev."Investors put a lot of weight on how well an organisation is led," he said."Right now leadership is a matter of uncertainty for the bank and we're going to have to wait until next year before we know who will be next to lead the business."Rice, a forty-year veteran of the funds management industry, said it was critical the board look outside the bank to identify the next CEO."CBA has been starting to smell for the last year," he said."Investors are assessing that risk in the CBA is increasing while it remains static for its three major rival banks."I'm very concerned that the problems will take some time to turn around."Despite the recent underperformance in its share price,