The decline followed a sharp rise in the regular session sparked by positive initial results in an early-stage trial of a gene therapy for blood diseases including sickle cell anemia.
CRISPR rose $9.93, or 17%, to $68.46 in regular trading.
The stock fell $3.46, or 5%, to $65.00 in after hours action.
Underwriters of the share offering will have a 30-day option to purchase up to 637,500 additional common shares.
Elsewhere, biotech stocks were generally higher Tuesday.
Winners outpaced losers by more than 4 to 1 in the sector.
Among other notable biotech movers Tuesday, Athenex shares (ATNX) - Get Report rose $4.08, or 35.66%, to $15.52. The company said it will present results of a phase III test of its Oral Paclitaxel treatment for breast cancer at the San Antonio Breast Cancer Symposium next month.
Novartis is a holding in Jim Cramer's Action Alerts PLUS member club.