You can't buy a stock "on margin" of any company selling for less than $5, I believe it's an exchange or regulatory rule but it has to be enforced by the broker. They make very few exceptions. It actually cuts both ways since you have to short in a margin account it is harder to short a stock selling for less than $5. You can still short a stock for less than $5 but you have to have enough collateral in a margin account to cover the full cost of the stock, and you can buy it in a margin account but you have to have enough collateral to cover the full cost.
BTW, this also helps to explain why so many shorts jump on EXEL when it crosses above $5 (not to mention the convert hedge). Most firms require a stock to trade above $5 for an extended period ( usually 1 or 2 weeks) before they allow margin traders to buy or short "on margin" (i.e. less collateral than the full cost of the stock). Combine this with the convert hedge, the stock will have massive resistance at the 5.30-5.50 range. Continued good news might get us there but the reality of trading will make it very tough to get through that zone of resistance.