In contrast to the cblC defect which presents exclusively as combined hyperhomocysteinaemia with MMA-uria, cblD patients can have three distinct biochemical phenotypes: isolated hyperhomocysteinemia (cblD-HC), isolated MMA-uria (cblD-MMA, beyond the scope of these guidelines), or combined hyperhomocysteinemia and MMA-uria (cblD-MMA/HC). Here, CBLC is linked to hyperhomocysteinemia.