The Company considers only those investments which are highly liquid, readily convertible to cash, and that mature within 90 days from the date of purchase to be cash equivalents. At March 31, 2024 and December 31, 2023, cash equivalents were comprised of money market funds, commercial paper, and other debt securities with maturities less than 90 days from the date of purchase.

Restricted cash as of March 31, 2024 included security for a stand-by letter of credit issued in favor of a landlord for $669,900 of which $284,950 was classified in current assets and $384,950 was classified in noncurrent assets as of March 31, 2024.

As of March 31, 2024, the Company’s cash and cash equivalents held in the United States ("U.S.") was approximately $9,274,000 and approximately $4,829,000 of cash was held in its subsidiaries in the United Kingdom and Australia. As of December 31, 2023, all of the Company’s cash was held in the U.S., except for approximately $3,772,000 of cash which was held in its subsidiaries in the United Kingdom and Australia.

Our foreign subsidiaries in the United Kingdom and Australia may qualify for refundable research and development tax credits in the form of cash that were earned on certain research and development expenses incurred primarily outside of the U.S. In the period ending March 31, 2024, the Company received refundable research and development credits from foreign tax authorities of approximately $2,543,000 that were recorded in other income (expense), net. No future conditions impact the recognition of these tax credits.