B-type cyclin involved in DNA replication during S phase; activates Cdc28p to promote initiation of DNA synthesis; functions in formation of mitotic spindles along with Clb3p and Clb4p; required with CLB5 for premeiotic DNA replication and activation of the meiotic DNA replication checkpoint; most abundant during late G1; CLB6 has a paralog, CLB5, that arose from the whole genome duplication