question: How is CBX1/M31 related to position-effect variegation?
answer: M31 is a heterochromatin component, that is concentrated in the XY body during spermatogenesis. M31 overexpression has two contrasting effects which are dependent on chromosomal context: (i) it enhanced PEV in those lines with centromeric or pericentromeric transgene locations; and (ii) it suppressed PEV when the transgene was non-centromeric.