CAL.20C, also known as lineage B.1.429, is defined by five distinct mutations (I4205V and D1183Y in the ORF1ab-gene, and S13I, W152C, L452R in the spike proteins S-gene), of which the L452R (previously also detected in other unrelated lineages) was of particular concern. CAL.20C is possibly more transmissible, but further study is necessary to confirm this. It was first observed in July 2020 by researchers at the Cedars-Sinai Medical Center, California, in one of 1,230 virus samples collected in Los Angeles County since the start of the COVID-19 epidemic. It was not detected again until September when it reappeared among samples in California, but numbers remained very low until November. In November 2020, the CAL.20C variant accounted for 36 percent of samples collected at Cedars-Sinai Medical Center, and by January 2021, the CAL.20C variant accounted for 50 percent of samples. In a joint press release by University of California, San Francisco, California Department of Public Health, and Santa Clara County Public Health Department, the variant was also detected in multiple counties in Northern California. From November to December 2020, the frequency of the variant in sequenced cases from Northern California rose from 3% to 25%. In a preprint, CAL.20C is described as belonging to clade 20C and contributing approximately 36% of samples, while an emerging variant from the 20G clade accounts for some 24% of the samples in a study focused on Southern California. Note however that in the US as a whole, the 20G clade predominates, as of January 2021. Following the increasing numbers of CAL.20C in California, the variant has been detected at varying frequencies in most US states and small numbers have been detected in other countries in North America, Europe, Asia and Australia.