Unknown bga part identification

Hi all, I am searching for the part with the marking R21 to make a substitution for a broken board. I identified the LC7 (analog devices, LDO, ADP121-ACBZ18R7, 1.8V output) and the PD1S 701 part (PESD1USB30, ESD protection for differential data lines). The IC is a BGA with a rectangular footprint and appears to have 4 BGA balls (not 100% sure about this, though). The board is essentially a USB fanout: D+/D-, 5 V, and GND come in on one side, providing six pins to connect to the target device. Four of these pins are connected to the USB lines, and the remaining two unknown ones are connected to this mystery IC. The IC appears to be powered with the 1.8 V LDO and connected through the ESD protection to these mystery lines.

It would be great if someone had some pointers!

Thanks!