Castor (Cas) is a temporal transcription factor expressed in NB5-6 neuroblasts. It activates the transcription factor Collier (Col), which then promotes the expression of Apterous and Eyes absent (Ap/Eya). In Tv1, Col and Ap/Eya form a feedforward loop to activate the neuropeptide called Nlpl1. This results in the neuroblast producing a Tv1 neuron (top). In Tv2/3 neurons, Cas activates a feedforward loop consisting of Squeeze (Sqz), Nab and Seven up (Svp) that inhibits Col, blocking the feedforward loop that leads to Nplp1 expression and allowing the establishment of the Tv2/3 identity (middle). In Tv4, Cas promotes the gradual expression of the next temporal transcription factor, Grainyhead (Grh), which inhibits Cas and Svp (thus blocking Tv2/Tv3 specification), while allowing the formation of a feedforward loop involving Ap/Eya and BMP that promotes the expression of the neuropeptide FMRFa. The neuroblast now produces a Tv4 neuron. Stratmann et al. explored the role of another temporal transcription factor, Krüppel (Kr), during the Cas window. They found that, at first, Kr inhibits Svp: this allows the establishment of the Col>Ap/Eya>Nplp1 feedforward loop that leads to the formation of Tv1 neurons (top; key players shown in blue). Later in the window, the Cas>Sqz>Nab feedforward loop inhibits both Kr and Col, as does Svp, which allows Tv2 and Tv3 neurons to form (middle; key players shown in red). Later in the window, a putative factor helps Grh to repress Svp, Kr remains inhibited by Sqz and Nab, and Grh promotes the expression of FMRFa that specifies Tv4 (bottom; key players shown in green). BMP: bone morphogenetic protein.