The following commentary comes from an independent investor or market observer as part of TheStreet's guest contributor program, which is separate from the company's news coverage.NEW YORK ( InvestorPlace) -- Some of the hottest penny stocks to buy now are in the biotech sector. Start-ups and early-phase biotechnology companies can provide explosive profit potential for investors who are in on the ground floor of a game-changing medical technology or blockbuster drug. That said, finding hot penny stocks in the biotech sector is no easy task. There are lots of micro-caps and pink sheet stocks out there that wreak havoc on the portfolios of penny stock investors. But buying penny stocks doesn't have to be an all-out gamble -- by screening for investments with a decent market cap ($100 million or more) and a listing on a major exchange, you can find ultracheap stocks that are very fast movers but considerably less risky. Here are my favorite four biotech penny stocks to buy now: YM BioSciences ( YMI): The first stock on the list is biopharmaceutical company YM BioSciences, which develops products for the treatment of cancer. In the last 12 months, YMI stock has gained 63%. More recently, YMI is up 8% year to date and is trading near its 52-week high of $2.52. Zalicus ( ZLCS): Since the start of 2011, has watched its stock value rise 44%, compared with much smaller gains by the broader markets. This biopharmaceutical company, which focuses on the treatment of pain and inflammation, has experienced stock growth of 70% since last April. Buy this stock with a 52-week range of $1.01 to $2.72. Zalicus is one of InvestorPlace.com's 10 Best Stocks for 2011. AEterna Zentaris ( AEZS): Drug development company AEterna Zentaris specializes in oncology and endocrine therapy. Since the start of 2011, this penny stock has raised its stock value 8%. Investors are even more thrilled to see 52-week growth of 87%.