This is a family of cell surface proteins found in Mycobacterium with no known function [1].

1: Mycobacterium tuberculosis septum site determining protein, Ssd encoded by rv3660c, promotes filamentation and elicits an alternative metabolic and dormancy stress response. England K, Crew R, Slayden RA; BMC Microbiol. 2011;11:79. PMID:21504606