5000000 Ordinary Shares Bofa Securities Piper Sandler

TABLE OF CONTENTS Registration No. 333-232677 Filed Pursuant to Rule 424(b)(5) SUBJECT TO COMPLETION, DATED NOVEMBER 18, 2020 s e e e h h i t t t i r y PROSPECTUS SUPPLEMENT r e b u d c (To Prospectus dated July 19, 2019) e n e v s u i t t e c s n e e f e f h t e m 5,000,000 e l t d l a e e t r s s a l o c n t e o r i d t e f a n f r t e o s e i b n g a s e r t a o h e n s v Ordinary Shares i e e t i r t c i a e r f s u f u c e t e c s n e a . e p We are offering 5,000,000 of our ordinary shares, nominal value $0.00003881 per ordinary share. d s s o e e o t t r h t t i p g Our ordinary shares are listed on the Nasdaq Global Select Market under the symbol “MGTX.” On n o m e i t r t s e November 17, 2020, the last reported sale price of our ordinary shares on the Nasdaq Global Select Market a a g l p b n e i t was $15.14 per ordinary share. r t g o a , n l n s i e s u y r i t We are an “emerging growth company” and “smaller reporting company” under the federal n t c e a n e l securities laws and are subject to reduced public company disclosure standards. See “Summary — Emerging p e a p s s m m o Growth Company Status.” r o e r t o c p a c r t a s e e f Investing in our ordinary shares involves risk. See “Risk Factors” on page S-8 of this prospectus s e f n a h o o b t i supplement, on page 5 of the accompanying base prospectus and our filings with the Securities and Exchange t e g a d h r t Commission, or the SEC, that are incorporated by reference herein and therein to read about factors you should n n t i a s e y i r t consider before investing in our ordinary shares. n g e n a e h e r p w m m A e n l o . o c p d i Per Share Total c t p e a c u g i s n e d Public offering price $ $ a s s h i t h u r t c Underwriting discounts and commissions(1) $ $ u d c j e e n b y a p Proceeds, before expenses, to us $ $ s n y t o a a n r e n p m i m y d s e r (1) We refer you to “Underwriting” beginning on page S- 13 for additional information regarding e l n a i a p t n i underwriting compensation. p i e r t u u m s e i c l l e s p e The underwriters may also exercise their option to purchase up to an additional 750,000 ordinary s r u t m p e c shares from us, at the public offering price, less the underwriting discount, for 30 days after the date of this o s e s c e i p t h prospectus supplement. h s t o o T n y r . u p s n i Neither the SEC nor any state securities commission has approved or disapproved of these securities or b o , y i r o d determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any s t a e s i r n d i representation to the contrary is a criminal offense. e n m f m f e i m o l m o e The ordinary shares will be ready for delivery on or about , 2020. a n r C a p s a e g s , g i n i 3 n h t t a i 3 c h 9 i n c 1 l i Joint Book-Running Managers x o f n s E o o t t i d o t c n BofA Securities a n a A e m s s r r e e a o i i f Piper Sandler t t e i i n r r i w u u e c c d Evercore ISI h e e n T S S a Barclays Chardan The date of this prospectus supplement is November , 2020. TABLE OF CONTENTS TABLE OF CONTENTS Page Prospectus Supplement About This Prospectus Supplement S-1 Where You Can Find More Information; Incorporation by Reference S-2 Summary S-4 The Offering S-7 Risk Factors S-8 Cautionary Note Regarding forWard-Looking Statements S-9 Use of Proceeds S-10 Dividend Policy S-11 Dilution S-12 Underwriting S-13 Material Cayman Islands Tax Considerations S-20 Material United Kingdom Tax Considerations S-21 Material U.S. Federal Income Tax Consequences to U.S. Holders S-24 Legal Matters S-29 Experts S-29 Page Prospectus About This Prospectus 1 Where You Can Find More Information; Incorporation by Reference 2 The Company 4 Risk Factors 5 Use of Proceeds 6 Description of Share Capital 7 Description of Warrants 16 Description of Units 17 Global Securities 18 Plan of Distribution 22 Legal Matters 24 Experts 24 S-i TABLE OF CONTENTS ABOUT THIS PROSPECTUS SUPPLEMENT This prospectus supplement is part of a registration statement that we have filed with the SEC utilizing a “shelf” registration process, and relates to the offering of our ordinary shares. Before buying any of the ordinary shares that we are offering, we urge you to carefully read this prospectus supplement, the accompanying base prospectus and all of the information incorporated by reference herein and therein, as well as the additional information described under the heading “Where You Can Find More Information; Incorporation by Reference” in this prospectus supplement. These documents contain important information that you should consider when making your investment decision. We provide information to you about this offering of our ordinary shares in two separate documents that are bound together: (1) this prospectus supplement, which describes the specific details regarding this offering; and (2) the accompanying base prospectus, which provides general information, some of which may not apply to this offering. Generally, when we refer to this “prospectus,” we are referring to both documents combined. If information in this prospectus supplement is inconsistent with the accompanying base prospectus, you should rely on this prospectus supplement. To the extent there is a conflict between the information contained in this prospectus supplement, on the one hand, and the information contained in any document incorporated by reference in this prospectus supplement, on the other hand, you should rely on the information in this prospectus supplement. If any statement in one of these documents is inconsistent with a statement in another document having a later date — for example, a document incorporated by reference in this prospectus supplement — the statement in the document having the later date modifies or supersedes the earlier statement. Neither we nor the underwriters have authorized anyone to provide any information or make any representations other than those contained or incorporated by reference in this prospectus supplement, the accompanying prospectus or any free writing prospectus that we have authorized for use in connection with this offering. We take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may give you. Neither we nor any of the underwriters are making an offer to sell these securities in any jurisdiction where the offer or sale is not permitted or in which the person making that offer or solicitation is not qualified to do so or to anyone to whom it is unlawful to make an offer or solicitation. You should assume that the information appearing in this prospectus supplement, the accompanying base prospectus, the documents incorporated by reference herein and therein and any free writing prospectus that we have authorized for use in connection with this offering, is accurate only as of the date of those respective documents. Our business, financial condition, results of operations and prospects may have changed since those dates. You should read this prospectus supplement, the accompanying base prospectus, the documents incorporated by reference herein and therein and any free writing prospectus that we have authorized for use in connection with this offering, in their entirety before making an investment decision. We are offering to sell, and seeking offers to buy, ordinary shares only in jurisdictions where offers and sales are permitted. The distribution of this prospectus supplement and the offering of the ordinary shares in certain jurisdictions may be restricted by law. Persons outside the United States who come into possession of this prospectus supplement must inform themselves about, and observe any restrictions relating to, the offering of the ordinary shares and the distribution of this prospectus supplement outside the United States. This prospectus supplement does not constitute, and may not be used in connection with, an offer to sell, or a solicitation of an offer to buy, any securities offered by this prospectus supplement by any person in any jurisdiction in which it is unlawful for such person to make such an offer or solicitation.

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