Exhibit 10.31

FIRST ADDENDUM TO STANDARD INDUSTRIAL/COMMERCIAL SINGLE-TENANT LEASE – NET DATED
AUGUST 15, 2014 BY AND BETWEEN GEIGER COURT, LLC, AS LESSOR AND EXAGEN
DIAGNOSTICS, INC. AS LESSEE, FOR THE PREMISES LOCATED AT 1221 LIBERTY WAY,
VISTA, CALIFORNIA.

Lessor and Lessee mutually agree to amend the lease as follows:

Paragraph 1.3 (Term):
The Lease Term will be extended to the new Expiration Date of January 31, 2021

Paragraph 50 (Rent Schedule):
Base Rent shall be as follows:

DatesBase RentFebruary 1, 2018 - January 31, 2019$19,308.96 per monthFebruary 1,
2019 - January 31, 2020$19,888.23 per monthFebruary 1, 2020 - January 31,
2021$20,484.88 per month

Paragraph 58 (Existing Furniture):

Replace “Lessee may use furniture for $0.01/SF per month during the Lease Term”
with “Lessee may use furniture for no charge during the Lease Term”.

Building Improvements:

Landlord will replace outdated thermostats in building with new advanced
thermostats. Landlord will replace manual rocker light switches in offices,
conference rooms, copy room and break room with motion sensing automatic on and
off switches. Tenant will coordinate the installation of these improvements
remit direct costs to Landlord for reimbursement.
1.Landlord to pay initial $2,500. Tenant to pay next $2,500. Cost to be split
50/50 over $5,000.
2.Landlord cost capped at no higher than $5,000.
3.All work to be completed prior to end of 2017.

Lessee will be responsible for any other work or modifications to HVAC systems
as determined by Lessee. Upon execution of this Addendum, Lessee shall be
responsible for contracting direct with an HVAC vendor for maintenance and
repairs and have full control over the HVAC systems.

All other terms and conditions of the existing Lease shall remain in full force
and effect.

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LESSOR:

LESSEE:Geiger Court, LLC

Exagen Diagnostics, Inc.

By: /s/ Hal Tilbury

By: /s/ Ron Rocca

By:

Title: CEO

Date: 8/2/2017

Date: 8/2/2017