In light of the numerous studies on the detection of target letters among adults, it is generally accepted that the missing-letter effect depends both on a given word's frequency in its language and on its role (function vs. content) in a sentence. Following a presentation of several models explaining these observations we analyze the results of a letter-detection task given to 886 French students from kindergarten to second grade. The purpose of the present study is to determine the moment when the sensitivity to content/function word distinction emerges. The results of this study reveal that even if word frequency plays a role in letter detection, the emergence of an ability to extract sentence structure, along the lines of the structural model of reading, is significantly linked to the initial stages of explicit reading instruction. (Contains 2 tables, 2 figures, and 4 notes.)