Shares of SVB Financial Group, known as Silicon Valley Bank, tumbled for a second day Friday and weighed on the whole banking sector again on fears more banks would incur heavy losses on their bond portfolios.

SVB's CEO Greg Becker held a call with clients Thursday evening to calm their fears after a 60% tumble in the stock, CNBC has learned. The shares were down another 45% in premarket trading Friday.

The SPDR S&P Regional Banking ETF was off another 1.5% Friday following an 8% tumble on Thursday. The Financial Select SPDR Fund was down by 1.25% following a 4% decline on Friday. Signature Bank, which does a lot of business with the crypto sector, was off 4% in premarket trading following a 12% tumble Thursday. First Republic Bank was of 3% following a 17% tumble on Thursday.

Major banks were also under pressure with JPMorgan Chase losing another 1% early Friday after tumbling 5% on Thursday.

"Current pressures facing SIVB are highly idiosyncratic and should not be viewed as a read-across to other banks," wrote analysts Manan Gosalia and Betsy Graseck with Morgan Stanley in a note Friday. "The funding pressures facing SIVB are highly idiosyncratic and should not be viewed as a read-across to other regional banks."

SVB said in a letter from Becker on Wednesday that it sold "substantially all" of its available-for-sale securities made up of mostly U.S. Treasurys. The bank said it was looking to raise $2.25 billion between common equity and convertible preferred shares. Investment fund General Atlantic has already committed to contribute $500 million.

The bank also previously reported more than $90 billion in held-to-maturity securities, which wouldn't necessarily incur losses unless it was forced to sell them before maturity to cover fleeing deposits. As the Federal Reserve consistently raises interest rates, it is lowering the value of Treasuries. For example, the iShares 20+ Treasury Bond ETF, which is made up of longer maturity Treasuries, is down 24% in the last 12 months.

Investors are also worried about lack of support from Silicon Valley Bank's funding base of tech start-ups, an area hit hard from the slumping stock market and surging rates. Peter Thiel's Founders Fund and other large venture capital firms asked its companies to pull their funds from SVB, Bloomberg News reported.

"Falling VC funding activity and elevated cash burn are idiosyncratic pressures for SIVB's clients, driving a decline in total client funds and on-balance-sheet deposits for SIVB," wrote the Morgan Stanley analysts. "That said, we have always believed that SIVB has more than enough liquidity to fund deposit outflows related to venture capital client cash burn."

SVB had a market value of $16.8 billion to end last week. On Thursday, the bank was worth $6.3 billion with that value set to drop even more when trading begins Friday.

This is a developing story. Check back for updates.