A couple of weeks ago, scientists from Australia and Spain published a paper in the Journal of Agricultural and Food Chemistry in which they reported finding a gene (and characterized the transcribed protein) in a particular yeast strain that made the resultant beer foamier, the first time scientists have ever done so. While knowing the identity of a particular yeast gene that promotes foaming isn't going to be of much use to most homebrewers -- unless you are a dab hand at creating recombinant yeast strains, in which case, have at it -- it's of interest to the greater brewing industry because understanding the creation and retention of a head on a glass of beer is an important quality control technique. The quality of foam on a pint is one of those things that consumers notice and subsequently use to, consciously or not, judge your beer.
So, what is beer foam? At its most basic, beer foam is a thin film of liquid, stabilized by various molecules including hop-derived organic acids and glycoproteins, around a volume of gas. Those glycoproteins--proteins with attached sugar groups--come in many forms, but today we are primarily interested in those called mannoproteins. That is, each protein chain has many mannose--a six-carbon sugar very similar to glucose--molecules attached to it.