EXHIBIT 5.1
 
                                  [Letterhead]
 
                                          August 4, 1998
 
Genus, Inc.
1139 Karlstad Drive
Sunnyvale, California 94089
 
        Re: Registration Statement on Form S-3
 
Ladies and Gentlemen:
 
    We have examined the Registration Statement on Form S-3 to be filed with the
Securities and Exchange Commission (the "Registration Statement"), in connection
with the registration under the Securities Act of 1933, as amended, of a shelf
offering of 1,676,877 shares of your Common Stock (the "Shares") issued upon
conversion of Series A Convertible Preferred Stock and to be issued upon the
conversion of issued and outstanding shares of the Series B Convertible
Preferred Stock (the "Series B Stock"), exercise of Warrants to Purchase Common
Stock (the "Warrants") and dividends on the Series B Stock in accordance with
the Articles of Incorporation as currently in effect. As your counsel, we have
examined the proceedings proposed to be taken in connection with the sale and
issuance of the above-referenced securities.
 
    In our opinion, the shares issued upon conversion of the Series A
Convertible Preferred Stock are legally and validly issued, fully paid and
nonassessable, and the Shares to be issued upon conversion of the Series B
Stock, the exercise of the Warrants and as dividends on the Series B Stock will
be legally and validly issued, fully paid and nonassessable.
 
    We consent to the use of this opinion as an exhibit to the Registration
Statement, and further consent to the use of our name wherever appearing in the
Registration Statement, including the Prospectus constituting a part thereof,
and any amendment thereto.
 
                                          Very truly yours,
 
                                          WILSON SONSINI GOODRICH & ROSATI
                                          Professional Corporation
 
                                          /s/ WILSON SONSINI GOODRICH & ROSATI