This article is from the Static Electromagnetic Fields and Cancer FAQ, by John Moulder email@example.com and the Medical College of Wisconsin with numerous contributions by others.
In general, static magnetic fields do not appear to have this type of
epigenetic activity. There are a few studies that suggest that static
magnetic fields might enhance the effects of other genotoxic agents, but
none of these studies has been replicated.
Three studies [14,30,31] have found that 140-3700 mT static fields do
not enhance the mutagenic effects of ionizing radiation; but a fourth
study  reported that 1100-1400 mT static fields caused a slight
increase in the number of chromosome aberrations produced by exposure to
high doses of ionizing radiation, and a fifth study reported that a 4000
mT field slightly increased radiation-induced cell killing . Repair
of radiation-damage was reported not be affected by a 140 mT field ,
but to be inhibited at 4000 mT . Kale & Baum  reported that
1300-3700 mT static fields did not enhance the mutagenic effects of a
known chemical genotoxin.
Two studies [35,36] found that 150-800 mT static fields did not enhance
the development of chemically-induced mammary tumors, but a third study
 reported that a 0.02 mT static field did enhance the development of
chemically-induced mammary tumors.