FAT1 and neural tube defect: Although mutations of the family member Fat4 do not cause NTD, and Fat4 binds different actin-regulating and junctional proteins than Fat1, they form heterodimers, and on some strain backgrounds, the combination of homozygous null mutations at both Fat1 and Fat4 cause increased frequencies of exencephaly compared with Fat1 alone [145]; this is an example of the genetic complexity of NTD etiology.