1(a) Name of Issuer: Mersana Therapeutics, Inc.
1(b) Address of Issuers Principal Executive Offices:
840 Memorial Drive, Cambridge, MA 02139, United States of America
2(a) Name of Person Filing: BB Biotech AG (BB Biotech) on behalf of its wholly-owned subsidiary, Biotech Target N.V.
(Biotech Target)
2(b) Address of Principal Business Office or, if none, Residence:
BB Biotech AG: Schwertstrasse 6, CH-8200 Schaffhausen, Switzerland
Biotech Target N.V.: Ara Hill Top Building, Unit A-5, Pletterijweg Oost 1, Curaçao
2(c) Citizenship: BB Biotech AG: Switzerland
Biotech Target N.V.: Curacao
2(d) Title of Class of Securities Common Stock, par value $0.0001 per share
2(e) CUSIP Number 59045L106
If this statement is filed pursuant to Rule 13d-1(b), or 13d-2(b) or (c),
check whether the person filing is a:
a. [ ] Broker or Dealer registered under Section 15 of the Act.
b. [ ] Bank as defined in Section 3(a)(6) of the Act.
c. [ ] Insurance company as defined in Section 3(a)(19) of the Act.
d. [ ] Investment company registered under section 8 of the Investment Company Act of 1940.
e. [ ] An investment adviser in accordance with Rule 13d-1(b)(1)(ii)(E);
f. [ ] An employee benefit plan or endowment fund in accordance with Rule 13d-1(b)(1)(ii)(F);
g. [ ] A parent holding company or control person in accordance with Rule
13d-1(b)(1)(ii)(G);
h. [ ] A savings associations as defined in Section 3(b) of the
Federal Deposit Insurance Act (12 U.S.C. 1813);
i. [ ] A church plan that is excluded from the definition of an investment company
under section 3(c)(14) of the Investment Company Act of 1940;
j. [ ] Group, in accordance with Rule
13d-1(b)(1)(ii)(J).
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