Snape's eyebrows lifted in surprise at the news that Regulus, not Sirius, was the heir acknowledged by Grimmauld. "I suppose your earlier return is the only explanation that makes sense: that or perhaps the house has divined your intention to carry on the Black name, and Sirius' pledged lack of that intent."
Snape snorted affectionately at Regulus' claim to brilliance, muttering "Shameless as ever," under his breath as the other man continued to speak. "I think we should use other places than the Order safehouse. We need sacrificial properties: shacks in the middle of nowhere, places we could rig to blow sky-high without losing anything of value to us."
He smiled evilly. "That's where the tissue sample comes in. Such things have more uses than Polyjuice - though that's always a good option to keep in reserve. No, I wanted a tissue sample, so I could key a curse to Bella: if she does attempt to infiltrate Grimmauld place via floo, she won't live long enough to regret her folly."