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Typically we would look to use a molecular weight cut off (MWCO) close to 1/3 the size of the target protein. In this case that would make a MWCO of ~28k, the closest membrane to this is a 30k which would be therefore be the suggested membrane. In other circumstances when in between possible MWCOs, we might suggest to use the lower MWCO to ensure that maximum recovery is reached, however in this instance the difference from 28k to 30k is negligible and so 30k is a good choice. Some may recommend 1/6 the size or ½ the size of the target, however with the former you risk too slow concentration speeds and with the latter you risk losing some protein to the filtrate or membrane matrix. The other consideration is what type of membrane to use for your protein, e.g. PES, RC, Hydrosart or CTA. This largely depends on your protein properties, however both PES and RC are good with most proteins. If you wish to confirm which is best we would suggest testing with sample devices.