CUSIP No. 34983P104 13D Page 22 of 24 Pages

EXHIBIT 99.1

AGREEMENT

Pursuant to Rule 13d-1(k)(1) under the Securities Exchange Act of 1934, the undersigned hereby agree that only one statement containing the information required by Schedule 13D need be filed with respect to the ownership by each of the undersigned of shares of Common Stock of Forty Seven, Inc.

Date: January 10, 2019

CLARUS LIFESCIENCES III, L.P.

By: Clarus Ventures III GP, L.P., its general partner

By: Blackstone Clarus III L.L.C., its general partner

By: Blackstone Holdings II L.P., its managing member

By: Blackstone Holdings I/II GP Inc., its general partner

By: /s/ John G. Finley

Name:	John G. Finley
Title:	Chief Legal Officer

CLARUS VENTURES III GP, L.P.

By: Blackstone Clarus III L.L.C., its general partner

By: Blackstone Holdings II L.P., its managing member

By: Blackstone Holdings I/II GP Inc., its general partner

By: /s/ John G. Finley

Name:	John G. Finley
Title:	Chief Legal Officer

BLACKSTONE CLARUS III L.L.C.

By: Blackstone Holdings II L.P., its managing member

By: Blackstone Holdings I/II GP Inc., its general partner

By: /s/ John G. Finley

Name:	John G. Finley
Title:	Chief Legal Officer

BLACKSTONE HOLDINGS II L.P.

By: Blackstone Holdings I/II GP Inc., its general partner

By: /s/ John G. Finley

Name:	John G. Finley
Title:	Chief Legal Officer

BLACKSTONE HOLDINGS I/II GP INC.

By: /s/ John G. Finley

Name:	John G. Finley
Title:	Chief Legal Officer

[Forty Seven, Inc. - Joint Filing Agreement]

CUSIP No. 34983P104 13D Page 23 of 24 Pages

THE BLACKSTONE GROUP L.P.

By: Blackstone Group Management L.L.C., its general partner

By: /s/ John G. Finley

Name:	John G. Finley
Title:	Chief Legal Officer

BLACKSTONE GROUP MANAGEMENT L.L.C.

By: /s/ John G. Finley

Name:	John G. Finley
Title:	Chief Legal Officer

/s/ Stephen A. Schwarzman

Stephen A. Schwarzman

CLARUS VENTURES III, LLC

By: /s/ Robert Liptak

Name:	Robert Liptak
Title:	Manager

*

Nicholas Galakatos

*

Dennis Henner

/s/ Robert Liptak

Robert Liptak

*

Nicholas Simon

*

Kurt Wheeler

*

Scott Requadt

*By:	/s/ Robert Liptak
	Robert Liptak, as Attorney-in-Fact

This Agreement was executed by Robert Liptak on behalf of the individuals listed above pursuant to a Power of Attorney a copy of which is attached as Exhibit 99.2.

[Forty Seven, Inc. - Joint Filing Agreement]