BlackRock has joined the list of firms filing to adopt Vanguard’s ETF multi-share class structure, pitching the dual-class flexibility as bolstering ETFs’ asset base by yoking them to mutual funds within 401(k)s.

In a Wednesday filing with the Securities and Exchange Commission, the firm became the 33rd to apply for open-ended mutual funds to list ETF shares. The filing also seeks approval to list mutual fund shares of ETFs.

Sponsors including Fidelity, Franklin Templeton and T Rowe Price are among the others also to have petitioned the SEC to adopt the multi-class structure, which until May of last year was off-limits because of a now expired Vanguard patent. Major providers yet to file include Capital Group, Invesco, JPMorgan and State Street Global Advisors.