Kinetic nonlinear mixed-effects model fit by saem using nlmixr
Structural model:
d_parent/dt = - ((k1 * g * exp(-k1 * time) + k2 * (1 - g) * exp(-k2 *
           time)) / (g * exp(-k1 * time) + (1 - g) * exp(-k2 * time)))
           * parent
d_m1/dt = + f_parent_to_m1 * ((k1 * g * exp(-k1 * time) + k2 * (1 - g)
           * exp(-k2 * time)) / (g * exp(-k1 * time) + (1 - g) *
           exp(-k2 * time))) * parent - k_m1 * m1

Data:
507 observations of 2 variable(s) grouped in 15 datasets

Likelihood:
   AIC  BIC logLik
  2400 2468  -1184

Fitted parameters:
                   Est.     SE  %RSE    Back-transformed(95%CI) BSV(CV% or SD)
parent_0            100  0.685 0.682              100 (99, 102)          0.278
log_k_m1          -5.35 0.0812  1.52 0.00477 (0.00407, 0.00559)           13.7
f_parent_qlogis  -0.091  0.096   106       0.477 (0.431, 0.524)          0.320
log_k1            -2.76 0.0969   3.5     0.063 (0.0521, 0.0762)           29.9
log_k2            -4.49 0.0658  1.47   0.0113 (0.00991, 0.0128)           18.6
g_qlogis         -0.167  0.142  85.3        0.458 (0.39, 0.528)          0.339
sigma_low_parent   1.04                                    1.04               
rsd_high_parent  0.0531                                  0.0531               
sigma_low_m1      0.819                                   0.819               
rsd_high_m1       0.058                                   0.058               
                 Shrink(SD)%
parent_0              87.7%>
log_k_m1              44.8%>
f_parent_qlogis     -0.375%>
log_k1                6.76%<
log_k2                10.4%<
g_qlogis              20.3%=
sigma_low_parent            
rsd_high_parent             
sigma_low_m1                
rsd_high_m1