Acts as an E3 ubiquitin-protein ligase specific for ldb1, mediating ubiquitination and proteasome-dependent degradation of excess ldb1 in a RING-dependent manner. Does not degrade ldb1 bound to lhx1/lim1, nor lim1 itself and thus contributes to the establishment of proper ldb1-lhx1/lim1 stoichiometry and the formation of a ldb1-lhx1/lim1 complex. Interferes with Spemann organizer function and suppresses secondary axis formation induced by ldb1 and lhx1/lim1 (By similarity). S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Forms homodimers through the C-terminal region. The N-terminus interacts with the homeobox of LIM/homeobox factor lhx1/lim1, with lhx3/lim3 and lhx5/lim5, and with the N-terminus of ldb1 (By similarity). Shows overlapping expression with lhx1/lim1 and ldb1 in the gastrula mesoderm, and expression overlaps with ldb1 throughout early embryogenesis. After gastrulation, expression is gradually restricted to tissues originated from the ectoderm, the neuroectoderm, neural crest and epidermis, and subsequently to the neural tube as well as the head and tailbud region. Expressed both maternally and zygotically. Expressed at the cleavage stage, with expression levels remaining constant throughout early embryogenesis, with a slight increase at the late gastrula stage. Belongs to the RNF12 family.