Naturally Light Higgs Doublet in the Spinor Representation of SUSY   SO(10)
  Within the supersymmetric SO(10) GUT we explore the possibility of the light Higgs doublet being a member of the 16-dimensional spinorial representation. This fact is ultimately related with the assumption that the light matter (at least partially) resides in some of the tensor representations as well. Several interesting features emerge. First, provided that the same 16-plet is responsible for the breaking $SO(10) \rightarrow SU(5)$, the heaviness of the top can automatically follow from the field content at $M_{GUT}$, without need of any flavour symmetries. Secondly, the doublet--triplet splitting problem receives a new natural solution. In addition, a dimension=5 (Higgsino mediated) proton decay can be naturally suppressed. We construct explicit SO(10) models with the above properties, with most general superpotentials under the symmetries. 
Gia Dvali and Stefan Pokorski