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Shares of CRISPR Therapeutics AG  (CRSP) fell in after-hours trading Tuesday after the gene-therapy company announced a public offering of 4.25 million shares.

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. 

The iShares Nasdaq Biotech ETF (IBB) rose $2.21, or 1.99%, to $113.45.

Winners outpaced losers by more than 4 to 1 in the sector.

Among other notable biotech movers Tuesday, Athenex shares (ATNX) 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. 

Medicines Co. shares (MDCO) rose $11.56, or 19.71%, to $70.21 on reports it may be a takeover target for Novartis (NVS) .

Novartis is a holding in Jim Cramer's Action Alerts PLUS member club.