Shares of the gene-editing company Crispr Therapeutics (NASDAQ: CRSP) fell by more than 14% in pre-market trading today. The culprit?Crispr and its development partner Vertex Pharmaceuticals (NASDAQ: VRTX) announced after the bell Wednesday that the Food and Drug Administration (FDA) has placed a clinical hold on the Investigational New Drug (IND) application for their sickle cell disease product candidate, CTX-001.