At the level of a penny, which is what the minimum price change is on most US stocks, almost anything can affect a sophisticated market-maker or proprietary trader's pricing: information in the stock's own order book, the price of related securities globally like the same stock trading in foreign markets, prices of correlated stocks, index futures prices around the world, his or her risk exposure in that stock and others in a portfolio, foreign exchange and interest rate market shifts, news feeds, sentiment analysis of online sources like twitter, company exposure to oil or other commodity price changes, etc. etc.