(Reuters) -The U.S. Food and Drug Administration has placed a clinical hold on Gilead Sciences' ( GILD ) trials testing a combination of two of its experimental HIV treatments, the company said on Tuesday.

Shares of the company fell about 2% at $110.77 in early trading.

The company said the agency placed the trials on hold after some patients who received the drug combination were found to have low levels of a type of white blood cell called CD4+T-cell.

The trials testing the drugs, GS-1720 and GS-4182, that were placed on hold included two in the mid-to-late stage, while three more were in the early phase, the company said.